Cases
I carry a 3 x 5, and am happy with it, even though 1 butt slot, and 2 shaft slots are unused right now. The 3's cases seem to have more room in the pockets than 2 x 4's, and that is important to me.
I also have a soft case which I can get 2 cues in if I want to go light, which I don't have to do because my 3 x 5 has a good padded shoulder strap and good handles and doesn't feel 'heavy' to me.
Leather is nice looking and okay, but many players complain about the heaviness after they load them up. And many leather cases have leather shoulder straps that are not padded in some other way.
I have carried a Porper 4 x 8, loaded to Vegas for 10 days, and I will never do that again.
I tend to favor a 3 x ? over a 2 x 4.
1) Playing cue with 2 shafts is a necessity for a serious player.
2) A Break/jump cue is almost a necessity, and should be given serious consideration. I carry 2 shafts for my break cue, after which cue would most likely have damage occur to it considering the force used. And you sure don't want to have a break/jump shaft problem in the middle of a tournament. A separate jump butt pocket would be a requirement for the case.
3) You might also like to have 2 jump type cues, some players do (short vs long jumps), so an individual jump cue might also be carried.
4) A 2nd playing cue I view as strictly an option, and actually not needed when your 1st one has 2 shafts, although some players like a 2nd cue maybe for 1 pocket, 14.1, or snooker.
Considering the above:
1 and 2 require a 2 x 4.
1,2, and 3 require a 3 x 5 or 3 x 6.
1,2,3 and 4 require 3 x 6 minimum (2nd playing cue has 1 shaft), or a 3 x 7, or going to a 4 x 8.
so logically, the 3 x 5 or 3 x 6 seems to be the best choices for a serious player, and you can fill them up however suits you. And I did not even mention whether you trade or buy cues at tournament events or not.
If you are going to leather, and intend to be a serious player, the 3 x 5/6 would better suit your needs now and in the future, but pay attention to the inside construction (tubes ?, what kind ?, JP's or not ?, handles ?, padded shoulder strap or not ?, how heavy empty ?).
I have a friend that spent $1300 on a new leather case with real nice tooling on it, a 3 x 6, but does not use it. Why? because the outside oval shaped circumference is TOO BIG, making it bulky to carry besides being heavy when loaded. Lesson to learn = how well does the casemaker use the space, and is it a tight arrangement? Sometimes, casemakers buy their outer shells, and work with them as opposed to having the shell manufactured to their specs, this can be a big difference.
Another point to consider, Are you a person that wants to spend considerable money on your dream leather case and carry it for 20-30 years, or do you like to turn over your cases more often, say every 5 years, and get the new improvements to them which might be dictated by customers desires or changes in Pool?