Fire/Fire vs Fire/MM
At higher lvls /MM will be more survivable and more damaging. Also, it doesnt hurt to have a different type of dmg at your disposal. Psy is rarely resisted(even tho when it is the mobs are practically immune).
Either way you will be forced to be ducking in and out of melee to maximize your pbaoe's so my vote goes to the one that helps you live longer and keep pumping out the dmg
@Injenius~Virtue
Stonefather - 50 Stone/WP/Soul brute
Sable Affliction - Earth/Dark/Fire Dom
Wild Cipher - Beast/Time MM
Fire/Fire gives you better potential ranged single-target damage because Ring of Fire is just that good (same damage as Fire Blast with a shorter activation time). Fire/Fire also gives you potentially more PBAoE damage.
Fire/Mental gives you another ranged AoE attack (Psychic Scream), a different damage type to work with (handy especially against Unstoppable-popping foes), and a little extra survivability. Psionic Shockwave is nice, but it just can't compete with the multitude of point-blank options in */Fire.
There's really no right answer here. If you do wanna hang out in melee range, then there's little question that MM is the more survivable option. Even if we ignore Drain Psyche's regeneration buff, you have a considerable -recharge debuff attached to basically all of your powers, along with a chance to stun in Psionic Shockwave and the albeit spotty protection of World of Confusion (a nearly useless power that becomes surprisingly strong if you slot the Coercive Persuasion proc in it).
From my own play, I have always found /mental to be more survivable.
You have a couple of very similar powers
Buildup = Concentration
Ring of Fire ~= Subdual
Then the sets diverge with fire going more for damage and /mental going for light control and survivability
World of confusion vs Hot feet and or blazing aura.
Drain Psyche vs Consume
Psychic Shockwave vs Fire Sword Circle, combustion, burn.
Fire Sword vs Mind Probe and TK thrust.
These days if I were building a fire/fire I would be building for melee and aoe caps. Much of the new content is designed to ignore S/L defense that was the staple of many suishy builds.
Fires only real control-ish power is fear in hot feet and burn. /Mental has KB in tk thrust that will have bosses flying from you like they were shot from cannons, chance of stun in psychic shockwave, a moderate confuse in world of confusion if you slot it with the purple confuse set.
With /mental I wouldn't be entirely happy going with S/L defense but psychic scream lets you stand back and hit them with aoe damage from a distance when you need to, something that /fire doesn't offer.
I currently have a 50 Fire/Fire blaster and at one point had a 30 Fire/MM. I have recently pretty much given up on blasters but from my experience my Fire/Fire was a monster. With some IO's i felt like if things didn't die in less than 3 seconds something was wrong.
@Nova King
Subdual is a better situational supplement, though, in the sense that its damage type usually bypasses the foe's defenses in situations where your Fire damage is heavily mitigated. In almost any other situation, incorporating Subdual into your attack chain will lower your DPS considerably.
The rest of your post is spot-on.
So, my friend an I are bored as hell last night and get into an interesting discussion about the 2 secondaries when paired with a fire primary, and which would actually be able to clear mass groups and survive still pretty decently. My money was on the Fire/Fire thanks to the sheer amount of AOE's it had at it's disposal to just rip through mobs, thus the theory of you kill them before they kill you tactic. While he was more into the Drain Psyche to up your regen so you'd be able to live through the fight with ease.
So now here I am, rolling up a blaster and doubting my original theory on Fire/Fire being absolutely almighty over Fire/MM. So now the question is this, Are there more advantages/disadvantages to these that I'm not taking into account? Because from a fun standpoint, I don't care which one, I have my fire primary, thus I am happy. But I would like to have a secondary that will best complement my primary.
Not even gonna bother bringing up the epic/patron power pools. I don't even want to think of the options that are in those at the moment.