I have a 177 Hw95 and a 20 caliber R9. I love them both. I probably like shooting the 20 caliber a little more because it packs a little more punch. However there's not many pellet choices for 20 caliber. It's basically four domes that are sometimes rebranded so it may seem like more. Pellet availability has been tough lately which also makes it more costly.
If you are only gonna have one springer a 20 caliber R9 /Hw95 is a wonderful choice. Just stock up on pellets once you find out what it likes. The pellets may continue to be made for some time but it is a dwindling niche caliber that may see more supply issues.
If you don't shoot a lot that's not a problem. If you do, you might want to consider 177. At least there, there will be hundreds of different pellets made for the reminder of our lifetime. They will also be more readily available and thus cheaper.