I have an HW50s in .22 caliber. I also have an R7 in .177 caliber. I also HAD and R9 in .177 caliber, but later sold it.
You're talking about birds at a "maximum" of 25 yards, so there is no way that you would actually "need" anything more than the HW50s in either of the 2 calibers available.
My R7 is great, but there were a few times when I felt it was a little underpowered for squirrels. Especially with the velocity loss that takes place in Denver at 5,280 feet elevation.
That's why I got the 50s. I just wanted a little more "punch" and I really didn't need a lot more "range" either.
AoA chronied my particular Hw50s in .22 caliber at 540fps before they shipped it too me. I don't have a chrony, but I would guess that my 50s shoots the 14.3 grain, .22 pellets at almost EXACTLY the same velocity that my R7 spits out the little .177 8 grainers. So, in reality, that's quite a bit more "Whack" on the other end.
I've also shot Eric's HW50s that is in .177 caliber, when I was over at Kevin Lentz's house last October.
I felt practically zero twang, and virtually no vibration in Eric's .177 version nor in my .22 version. None.
The HW50s is very accurate and I do not find it to be "hold sensitive" at all. It is very easy to shoot well, even in box stock format.
Cocking effort is a lot more than the R7, however. Roughly 36 lbs of effort on the 50s, compared to roughly 20 lbs or so on the R7. It's not bad, but you WILL notice a difference, if you are shooting a lot of rounds over a short time period.
Both myself, and Erick, previously owned R9's. I "think" his was in .177 caliber, and I know that mine was. Both of us ended up selling our R9's because they felt very "hold sensitive" to us, and "vibrated" a lot, in stock form, at least in .177 caliber. And neither of us really needed the R9's increased horsepower.
Now, with all of that said, I would actually be very curious to see what an R9 would behave like, specifically in the .22 caliber, as it may be noticeably smoother, etc. than the .177 cal.
IMHO, the HW50s is almost as easy to shoot well, as the R7 is. I can't really say the same about the R9 in "untuned" condition. They can be made real sweet, but out-of-the-box, they can take quite a bit of practice to figure out exactly how they like to be held.
The 50s is kind of a "plain jane" looking HW springer. No checkering! What's up with that?
Cocking effort is practically as high as the R9, but with not nearly the same power output in stock form.
But, in the end, the HW50s is shorter, lighter, slightly easier to cock, extremely accurate, much less "hold sensitive" and with less "vibration" than most stock, R9's. And it still has PLENTY of power for anything out to at least 35-40 yards minimum, and probably more than that for most critters.
Hope that helps a bit.