Ok, not all back inflate BCs give you the ability to dive doubles, so just keep that in mind.
Second, yes, back inflate BCs will help a bit with trim, but I've never had a problem with my trim while in a jacket style BC. Better trim, along with buoyancy control, and everything else in diving comes with more practice, it isn't something you go out and buy.
Third, depending on the BC, switching from doubles to singles and back to doubles really isn't that much of a pain, and besides, how often are you going to be hitting the surface and trying to switch over to doubles in a matter of seconds? I don't see it happening.
Fourth, keep in mind that no matter what you buy the odds are you're going to want a new BC before you need a new BC as long as you take care of your gear.
Fifth, if you see doubles or anything more than just your average, 60 ft, look at the pretty fishies dive in your future, you might want to look at a BP/W setup. There are lots of people who make them, my preference is DSS, some like OMS, some like Halcyon, some like Oxycheq. I've honestly never heard or seen anything too bad about any of them except for price issues with one or two of them, and I tend to agree.
Sixth, its mostly about what is going to work for you. There isn't a BC (made by a reputable company) out there that is going to totally make or break you as a diver. Most of the time when I hear someone complain about a BC issue, the issue is actually operator error, not an equipment problem.
Finally, figure out what you want to do, when you want to do it, and how you plan to do it, then look at the BC that will work best for you.
I think I rambled about everything I wanted to ramble about. I'll probably think of something else later.
