Plainville can fit more tables with more room (without things getting awkward) than Abington can.
Also, Derek likes to pack things in a little tighter than I'm comfortable with.
The terrain thing is covered in a couple of other threads right now. Abington has 30+ of brand new, gorgeous, 40k buildings and ruins. They're awesome. Day to day, week to week 40k games should have zero problem whatsoever finding terrain in Abington. Getting 15 tables together without caravanning to Plainville and back is a bit tougher, that's all.