Two completely different things with two completely different roles in my opinion.
22LR allows you to work on practical things such as firing position, breathing, trigger work, etc... but lacks in significant recoil. It also does not allow you to work on your handloading skills. That's also a benefit, because sometimes you just want to get a lot of cycles and all you have to do is buy a case of ammo that shoots well. That last part can get really tough because the ammo is outside of your control.
The 300BLK has value, but getting the kind of precision I got from mine is not for the faint of heart if you're trying for subs. I gave up on the subs. There was always a significant cold bore shift. Supers were fun. Certainly great practice. So if you're going for supersonic, then I'd say yes, it has value... but ONLY if you are willing to do an all the way build. Going cheap will NOT serve you here. Tons of people out there with cheap 300BLK's that can't hold a candle to mine.
You can't "really" practice long range shooting with the 22lr. The ammo is just not consistent enough to work with those kinds of ranges and wind calls. It's mainly for 50yds and closer, working on breathing, trigger press, and various other finite technique. The 300blk with supers can be used at distance and help you work on wind calls so long as you get it shooting precisely enough. It will also allow some firing position proofing.
So really the answer is that the one is not the other. The 300BLK is a MASSIVELY more expensive road to take... because it's a real centerfire and must be treated as such by all accounts. The 22LR road is cheaper, in that you already have everything you need to do that road. Your present rifle setup will allow you to work on things... but just remember that rimfire is a whole discipline within itself due to the ammo considerations. They just behave differently.
