Assuming you have a filling station that does it, and most do now... 300 bar pressure. 232's are cheaper but the bottle will empty much slower for the same volume bottle. Also get one with a surface only valve, as the tests are wider apart.
There's no problem with filling a 200 bar gun from a 300 bar bottle, but you need to take just a bit of care to ensure you don't go past 200 bar fill... there's been a handful of bottles with fast flowing valves, but most are easy to control with a bit of care, or you can get a flow restrictor fitted if you really want a slow fill. Another hitch can be getting an adapter for your rifle, but you can get 232-300 bar shims for the tank's valve to solve this.
Size wise, it depends... I've got a 12L 300 bar which I decant into a 3L bar bottle for taking to comps. The big bottles are heavy but wont need filling often. The smaller bottles are light, but do (depending on how much you shoot). A 7L is a 1/2 way house...
Most dive shops charge the same for a 3L as they do a 12L bottle to fill...
Carbon fibre is a light solution allowing a big tank to weigh less than a 3L normal one. But you need to look into the retest/lifespan of them... Littlekev on here should know more about them. They're not cheap, but it's an option perhaps.
Bestfittings is somewhere i'd recommend off the top of my head, they've always given me good service.
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