Another Power Meter Question

I've done some research and I "think" i know the answer but wanted to be for sure.  I'm looking at getting a power meter but I have a road and a tri bike.  I tend to train more on my road bike for outside group rides but ride my tri bike for races and on the trainer.  

My question is, I'm looking at crank based power, but my roadie is SRAM GXP and the tri is BB30.  I believe there are adapters out there but I wasn't clear how exactly they work.  Is it better to get the adapter for the GXP or BB30?  If I do that would I be able to swap cranks out fairly easily?

Eric