You've pretty much got the right of it. Fire will have damage out the wazoo, MM will be a much more balanced character due to Drain Psyche.
Depends on your preference really. I personally find Blasters do more than enough damage so the utility that MM offers is much more appealing. If all you want to be is a glass cannon (can be somewhat remedied with IOs) then go with Fire.
Go with whatever appeals to you more. Both will have lots of firepower and can be airly sturdy built well.
For Rad, I think you want both the tier 1 and 2 blasts. On Corrs and Defenders X-ray is skippable but the Blaster version has different numbers. Cosmic Burst is made of win, Irradiate is lovely. N Bomb is fairly decent but very slow, which may or may not drive you nuts. Electron Haze is decent damage but a cone and has a chance for KB.
The rest is personal preference. The snipe is a snipe, the nuke is a crashy nuke.
Support Guides for all Corruptor secondaries and Fortunatas
The Melee Teaming Guide for Melee Mans
The Melee Teaming Guide for Melee Mans
My Rad/Fire/Mace is soft capped to S/L and is hugely fun. I took all the PBAoE powers in the primary and secondary. Having an AoE immob from Mace just makes the combination tastier and Scorpion shield gives you a nice S/L defense boost that doesn't trash your costume.
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I have a Rad/Fire that I PLd to 50 and sat on the sidelines for a loooong time. About 1-2 months ago I decided to go ahead and build him and play him. Because it was a pure PL build, I didn't really have a great idea of how to build it, so I blatantly stole Miladys Knight's build I found in the Blaster forum, in a post about Rad/Fire. I made some minor mods to take Cold Mastery instead of Mace (since I had not run the patron arc) and I am pleased with Cold and I don't imagine I'll switch to Mace even though I've unlocked those powersets by now. Cold gets the same s/l def numbers and has sweet, sweet Hibernate. There is the costume issue, but aesthetics aren't my thing.
I can't compare to MM, since I have no hand-on experience with that secondary. Based on the powers it seems it would be a different playstyle. Rad/Fire is simply hit Aim or BU, jump into the middle of a spawn and hit your pbaoes until everyone dies. If there are resilient bosses use some st attacks. MM may have some similarities, jump in for Drain Psyche, Irradiate and Psi Shockwave, but then jump out for cones.
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So I'm thinking of making a Rad/ Blaster who will spend a lot of its time in AoE. I know Fire could be a serious damage monster, but MM has some heals and other options for survivability. Really what I'm wondering is what do you all recommend?
Also after being here 7 years, I still haven't found a blaster that's right for me and I know nothing about rad/ or /fire. I'm wondering which attacks I should consider skipping. I suppose a few tips for MM would be nice also, though I'm a little more familiar with that set. Thanks!