What do you currently have ? I had an EB-12 which was great, I only upgraded to an ABM from it due to a good deal on the ABM, otherwise it would have done me fine. It gigged a few times with a MAG210 extension cabinet and I never had the volume past half.
As for whether the 12 or 15 is better - that's personal preference. It's usually said that a 12 will have better mids than a 15, with the 15 having better lows... but I reckon there isn't a huge difference.