It depends on whether you like offense or defense. 4-1/4" pocket makes it easier to remove balls, harder on cut shots. On Diamond rails the corner pockets play soft so they have to go to smaller pockets to make them play a little tougher on ob coming down the rails.
On Brunswick Super Speed, not sure how tough their pockets play, and what size pocket should be recommended. But if you have a deeper shelf then that could effect the size pocket that is desired.
I like deeper shelf vs. keep making the pocket smaller. I believe a deeper shelf can be done pretty easily by using fiberglass resin. Although I have never done it. Deeper shelf combined with lively rails and 4-1/2" pockets works, can go smaller on pockets if you want to get use to aiming at a smaller target. Definitely smaller pockets with Diamond rails.
I think Diamond rails are angled cut at the pockets to allow the ball to be pocketed easier vs. the cut on Brunswick, but have no specific knowledge to offer up.
Whitey
Boy, if there is one thing I hate and do not understand, is any advantage to a deeper shelf! It is the ONE THING I dislike about Diamond tables. Why on earth do they have that unusually deep shelf that can nearly hide a ball behind the pocket point??? Part of why they play so deep is that the eased corner of the slate cut at the pockets is a smaller radius ease than most other tables I believe, so when you look right at the lip, there is a bit more of a waterfall effect on other tables, compared to Diamonds, at least it seem to me that way.
On top of that, our local most popular installer/re-covering table mechanic seems to often leave a very subtle ridge right at the lip that I suspect is a little glue residual that gets up on the lip edge when they use it to help keep the cloth neat as it wraps down into the pockets. Sometimes you actually see a ball slightly roll back from the lip lol, so it has to be something! The same local table mechanic does nearly all the tables in my area. He is good and respected, but getting closer to retirement and maybe helpers are doing a little more than they should lol.
I like standard 4-1/2" myself for a balanced offense/defense game and a table that does not play any sort of gimmicky.
I notice for the big money at Buffalo's, they like well broken in cloth, which by itself makes the pockets play significantly tougher, even if they are not any tighter in actual width.