I recently discovered CoH and none of my characters is level 50 yet so bear in mind the following may be completely irrelevant at high level and with a better knowledge of the game.
Having said this, I have made a few sentinels, mostly playing around the themes you mentioned (fire/beam rifle coupled with bio/energy aura/electric aura) and here are my impressions:
- Primaries (fire and beam rifle): as much as I love tossing fire balls left right and centre, I have vastly preferred beam rifle: it feels more "organic" with skills chaining in a smoother way. I have also preferred BR for AE situation. Not that fire can't handle them, but I find the spread of disintegration coupled with the couple of skills I have much more efficient. At identical difficulty level and on the same mission, I struggled much less completing the mission with BR.
- Secondaries:
-- Bio: I've read everywhere bio is the be all end all of secondaries. It may be at high level but so far (all my sentinels are in the 20-30 range), it has been way more fragile than the 2 auras and had more energy management issues
-- Electricity / Energy: to be honest, my impression is that these 2 are almost identical main difference being one focuses on resistances (electric aura) and the other on defense (a lot) and resistances (a little bit). So far (again, this may change at high level or with slotting for those who know what they are doing, i.e. not me), energy aura feels much more sturdy than electric aura (i.e. defense seems so far to be a higher priority than defense).
One final thing I have noticed is I have tried with and without a "combat fly" option (fly, hover, etc). These powers have greatly improved my survivability. Although most ennemies seem to have a ranged attack, the ones who would primarily focus on melee (brawlers for instance) seem to have weak ones which I suspect is what lead to this improved survivability. I would strongly suggest you pick one (whichever you prefer, fly just sucked more energy than hover but this wasn't vastly noticeable (except on the bio builds)). They can be a bit annoying when you want to use cones but they have made overall my life easier.
I have personally settled on a BR/energy aura toon and am very happy with it. I'm not trying missions for 6 players (I keep that for my mastermind) but she breezes through missions in a fashion none of my other sentinels do.
Hope this helps.
Edit: Another thing to mention, I believe BR has a higher inherent accuracy than fire which in turns mean I have to slot one less accuracy enhancement in each power for comparable "to hit" result. This has left me with easier early slotting options on my BR sentinels. Once again, this may be moot at high level but from my newcomer's perspective, it certainly added to the overall enjoyment of the set 🙂