From my nrg/ice blaster, i went from flame to cold mastery.
From 41 i took flame due the hold power (as you, i lack holds, outside frozen touch). Its a great power, 1 of my most used powers post-41. fast animation, slotted good duration and recharge. As i moved on, the shield has great SL resist but i noticed i hardly face ice/fire damage. (council flametrower, nemesis thingie, carnie abit). Has a very cool look though 
As i teamed alot in +6 teams, i once had a fire/fire/fire blaster along with bonfire. I HATE THAT POWER. freaking knockback making the whole group spread out, tanking loosing agro fast and everyone in panic. yet, higher lvl mobs tend to knockdown instead of knockback, but 99% of the time i've seen it it was doing knockback. Solo its nice, but it cant beat my ice patch by far. As often you dont face more then 4 mobs, having both is kinda overkill. The only good thing i can say.. its ranged, as ice patch is PB based. Perhaps i used it wrong, or its a too situational power.. but as you have ice patch, i rather not pick it.
RoTP i was badly dissapointed. Yes it recharge fast, but the damage is puny and i was around someone with rez about 99% of the time. Its your personal rez power.. thats it. Specialy if the team move on to next group of tanker pulls it away.. it makes it kinda useless as it still incur debt.
So finaly at 49 i moved to Cold mastery. As i was mostly teaming having no hold wouldnt be that big problem. I took all 4 powers, you can read what they exactly do in another topic here. With /ice, its plain awsome. We did this AV mission, where 1 group got agro'ed and tanker was already facing the AV. I popped CE on, wacked snowstorm on a mob in the middle. Ran around a corner and put icepatch. Ice shield on (its defence based), 1 purple and waited.
No single mob was able to hit me much, they were cap slowed (iirc 90% is cap on recharge/slow), nonstop knockdown and their damage was lowered (abit) by CE. But.. 1 mob mezzed me. All toggle drop, Ice patch blinking! 1 breakfree and i popped hibernate. 30 seconds of immortality i regain full health, full endurance while mobs kept stupidy wacking me. Long enough for the tanker to take over the agro and i could drop my hibernate again. Snowstorm helps great on AV's too, the slow/recharge works greatly to reduce incomming damage to the tanker. Hibernate is up quite fast, making it also a secondairy Rest power.
Solo i loved the ranged AoE sleep. As its very big (iirc 20ft), i could sleep all mobs in range. Cant remember atm if it instant sleep a boss though. Once slept, i could pick each mob 1 by 1, putting frozen touch on those who would be tricky. Team its 'ok', great when accidently 2nd group gets agro or for a 'BE QUIET NOW' moment. As it still has a quite long recharge, i still love the power alot.
I stick with Cold now, it suits my nrg/ice more. The shield gives defence, wich helps abit more on mezz (miss = no mezz) then a resist shield. Although it looks kinda stupid on a female toon 
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well im a lowbie ice devices but ill be taking fire cos of the lovely little rez it has, and as a fire fire domon villains im taking bonfire and ofc shield. ok first of all bonfire is a great kB. BUT i only use it when im or the team is in danger ppl use it anywhere and everyhwere and moan about it how many times ive mad ea small camp fire to save me is beyound me i say fire cos of the rez n sheild. huibernate on cold is good but as a blaster if ur enemy surronds u 30 secs later u dead! and who knows if urll use it in time not to die. at least rez is a kb i think and a disorient- enoguh to run like chickens
Cold Mastery I'd go with:
Frozen Armour - it's not amazing in my opinion, but better than nothing. Probably worth the endurance it costs, just.
Snow Storm - great power, not only a slow but also -fly, which makes certain AVs a bit easier - but this is going to give you endurance problems (and unless you've spent an awful lot on IOs I'd imagine that's something you already have).
Hibernate - which I would use both as a panic button, and also probably for endurance regain on occasion.
I definitely wouldn't take an AOE sleep on a fire blaster.
From Flame Master I'd take:
Char - really good hold, can be combined with Freezing Touch to hold bosses. Just a good pick for a fire blaster with no other ranged control available.
Fire Shield - reasonably good resistance to smashing, lethal and fire - it'll take the edge off incoming damage.
Rise of the Pheonix if room in the build (although I wouldn't slot it). People do die from time to time, it's a good res, and it's very pretty.
I would not take Bonfire - scattering stuff everywhere doesn't really do much for your AOEs in your primary.
I'd take Flame Mastery on that out of those two, mostly because of the hold and the endurance issues Snow Storm is going to add to.
Edited to add:
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would like to know your impressions about Bonfire.
Is its knockback good as a protection? Not often
Does it affect most enemies? Yes
Is it very annoying for teammates? Incredibly
I think I havenever seen this power in action... I think there may be a reason for this...
The recharge does not look very long and the redius is pretty big... True
I already have a knockdown power with this character, Ice Patch from secondary, which is a power I love and enjoy very much... ... and bonfire will throw things off it. :P
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There aren't very many situations where it's useful, particularly not on a PvE build.
I am trying to modify a bit the build for my Fire/Ice blaster and need some information and advice.
Currently I am trying to decide between Flame mastery and Cold Mastery Epic, though my character is only level 30 (hey, I plan very ahead)
I would like to know your impressions about Bonfire.
Is its knockback good as a protection?
Does it affect most enemies?
Is it very annoying for teammates?
I think I havenever seen this power in action...
The recharge does not look very long and the redius is pretty big...
I already have a knockdown power with this character, Ice Patch from secondary, which is a power I love and enjoy very much...
Also, from Cold Mastery I am considering Snow Storm, because I think the -recharge is really a very good damage mitigation for all team.
I already have two -recharge powers: Shiver and Chilling Embrace.
Would the three of them be overkill?
Is there a cap for the -recharge effect?
Regarding the armors from both Epics...which one is better in your opinion?
I am pretty much decided to go Flame Mastery, because of the ranged hold that Cold Mastery lacks, but maybe the other powers make Cold appealing, I don't know...
Anyway, all advice and info will be welcome!
PS. Do not tell me "go look into Nofuture powers guide", because I already did. I do not need hard numbers info. I need to know the "feel" of those powers, something that maybe only someone that has played them can provide...