If line capacity isn't an issue I would go with the smallest conventional reel for your target fish species. If it's a non-level wind reel a smaller reel is easier to lay line with. It's also easier to palm/handle during a cast.
What's the reason for switching to conventional? Based on my experience, if you're only tossing 3-4oz weights bottom fishing you're better off with a spinning setup with 20# braid if distance casting is a factor in your decision. Especially on a 10-11' rod. If you want to gain distance the benefit to going conventional is if you toss 6oz+ on a regular basis. Better if you using 8oz+ bait and using a 12-13' rod. In my experience conventional work better with monofilament line. I personally use 17# mono and throw 6-8oz+ and occasionally 10oz as needed. If you're throwing bigger weight and baits it's easier to palm a conventional reel and get the added distance if you're a power caster. For me I feel a conventional reel is more compact and I have a better casting motion compared to a spinning reel. In theory there's less friction in a conventional setup where the spool is pushing line out at basically the same speed line is pulled out after the initial cast. Compare that to a spinning setup the line is always being pulled out. I will say that nowadays you can get some serious distance on a spinning setup with 20-30# braid but in the right hands a conventional can get slightly more distance even with a mono setup.