I've done a bit of research on this because I have a poly lower I want to make into a 22 platform. So, this my summary based on that research and my opinion. Opinions are like
a$$holes, everyone has one, so take what you want from the following.
- The BCG conversion kits are finicky, tend to jam, and can be ammo snobs. I won't use one of these.
- Dedicated 22 upper - Depending on the upper and what they use, they can be just as picky as a BCG, but typically have all the kinks worked out and can be a reliable platform. I've been waiting for the Nordic Components NC22 upper to be available since they use a bolt more like the 10/22 (square vs round) and appears to be designed to address the conversion issues.
- M&P 15-22 - A great little rifle, but different enough that if you are looking to replicate your AR platform, it won't be the same. Functionally they're close. By itself, I'd recommend it if you want to practice your sighting and whatnot. Also really fun for a steel challenge
Overall, you need to decide what your ultimate goals are and what bet matches that.