You answered your own question there: The powerplants are the same, and the velocities will be the same (in a perfect world, each gun is different, and pellet fit will have a big effect here). The marginal difference in barrel length won't have any noticeable effect on power. I've had and tuned both and noticed no difference that I could conclusively say was due to barrel length.
If you mean the HW77 (the long one, not the "K") it will be a noticeably heavier feeling and more unwieldy beast. Its definitely a more muzzle heavy gun. Personally, I didn't care for the one I had. The ONLY reason I could see getting one is if lower cocking effort was your main objective. The longer underlever will help with that. Otherwise, the HW77K is better in every way I can think of.
The 77K and 97 handle fairly similarly. I prefer the HW77K over the 97 for the ability to run open sights, and I like the balance a little better. Another little perk is it's easier to clean without having to fish the pull through or rod through the brake on the end of the 97.
As for caliber, it's an unpopular opinion, but I have never had a great shooting Weihrauch in .22, out of several I've tried. A lot of guys disagree here, and maybe they got lucky. But even most of those guys will admit that theirs are pellet picky at the least.
I've had multiples of 77, 77K, and 97 over the years. As well a whole bunch of other spring guns of all types. And I can say without a doubt that a 77K or 97 are my favorite and best shooting guns I've ever had. If you can handle the weight, I think you will be very pleased. Their weight is the only con I can think of, and on the other hand, it's one of their biggest pluses when it comes to accuracy.