You’ll likely get better advice than this but I would just pick one and go. I personally havent seen much of a difference from one primer to another. I just grab whatever the local store has at the time. I usually get match primers for target loads if available locally but many times they aren’t and I haven’t seen a noticeable difference switching back and forth. I suppose if a person was going for absolute best accuracy you could try a few different ones and run the loads through a chrono to see which one gives you the best es.
The Remington 7.5 are definitely tougher. I had trouble in my bench gun (6BRX) blowing a hole through the primer where the firing pin goes through the bolt face. I tried multiple primers and once I went to the Remington 7.5 it has never happened again.
Cci 450’s (and br-4) have the same amount of priming mixture in them as a cci 400. Only thing that makes them “magnum”is the thicker cup to withstand pressure. No change in the load is needed. The 450 and br-4 are identical by the way. Don’t waste money on the br-4.