It's all preference. I personally can't stand the yellow klingspor j-flexes because of the splice, and buy the blue hermes j-flexes in 120, 220, and 400, and then Awuko 800's, which are incredible, and I use more than any other belt. They're cheap, I buy 50 or so at a time.
Bear in mind that your 120, 220, and 400 j-flex belts will be used a lot for handle work also, and they wear out quickly on steel. I personally don't use the same ones I use for steel, on handle material, as they're dull and will burn and cause issues with handle material. Also this is the area where grinding steel, using a dull belt will fuck your grinds up as quick as anything.
I buy 5 or more times as many fine grit belts as I do course ceramics. I typically order 10- 50 or 60 grit, 10- 80 grit, and 10- 120 grit ceramics (usually 3m 984F cubitron ii's and the 967F for hard steel), and then 20- 120grit Hermes j-flex, 30- 220s, 400s, and 40- or 50- 800 grit Awuko j's.
The finer AO belts, only cut well for a short time, it's not an issue with the AO grain, it's just the fine belts lose cut and abrasive quickly. For a while I bought the 3m ceramic j-flexes in 120 and 220, but honestly they didn't last any longer in my opinion.
The j-flex belts are cheap though, usually $2-3 or so each. Use them like they're free is the mantra, especially the higher you go. Keep the ones you use for handle material seperate though, because especially 120, 220, and 400's can be used much longer on non-steel grinding, as long as that's all you use them for.
Also, I highly recommend getting a few cheap AO course grit belts for rough handle shaping. They're cheap, and will last a very long time if you use them only for this purpose, and will actually cut insanely better on handle material than your ceramics, which can serious screw up handle material, and the belts in this use.
I can't remember how long I've been using the same 36, 60, and 80 grit cheap AO belts for roughing and flattening handle material, just buy a few of each, then use the j-flexes you get for finish work.
You'll find out what works for your methods. I've tried most of the belts, and tricks out there. Cork belts, all the structured abrasives, have a whole stack of scotchbrite belts, don't use any of it very often for my style of grinding and finishing. I do occasionally use one of the really course gator (trizact) belts they came out with in the last couple of years, but don't find the rest of them to cut well enough or fast enough. Yeah they last a long time, and are probably great for certain satin belt finishes, but I rarely do belt finishes, and my goal is to get my grinds up to 800 with finished plunges as quick as possible, and only take a few minutes to hand rub to a fine satin hand finish.
Some of the things like trizacts, cork belts, leather belts, are expensive, so maybe try them out at a friend's first if you're on a budget, or when you've figured out what kind of grinds and finishes you like. Belts that work great for flat grinding non-hardened carbon steel, may not work worth a shit for hollow grinding hard stainless, etc.
