I'm getting a Schecter Omen Guitar & an Omen 4,5,6,or 8 String B for my twin girls. Which B should I get

I need to know what is the most common b guitar type and your opinion on what the best one would be and why. What other things do I need to buy besides the Amp? What Amp should I get? Hey thanks. Granny needs to know. I hope those kids like their Christmas presents.