Personally im excited for the firebug because of the possibility of the tier 4 weapon being the husk gun.
No no no, not the old husk gun the new one that shoots angry fireballs and can choose to project a liquid stream of hate to light things close to him on fire. Not to mention the addition of more area denial oriented weapons and possibly perk skills. I think what will determine my grater interest in firebug vs demo in all honestly is the perk tress and the 1-3 missing weapons for each perk.
Seriously, imagine a set of firebug skills where the entire point is a de-buff firebug. "Oh, what is that, the scrake, FP or boss is though? Let me wait until you are all ready, burn off his outer skin, armor etc and then you can pelt the crap out of him with explosives or massive amounts of concentrated fire.
Then again making the pipe bomb hat V2 with the new sticky C4 could be a barrel of laughs and if the missing weapon is a M32 + a really awesome grenade then i would be sold. I know i will end up playing all the perks but i have a feeling these 2 are going to be awesome.