I prefer a 2pc base as it gives easier access to the feed area,particularly when dealing with ammo problems in a blind magazine bolt action. Wider 1 piece rails make this a bit more difficult for my stubby fingers.
That said, the mounting/bedding and proper securing of the base screws is the most important part. I watched a nice EGW 20MOA mount come completely loose on a rifle at the range last time I went.