The glass quality is so high with the NF scopes that you could easily get by with the 3.5-15 but, my philosophy is that I can always turn the 22 down but I can't turn the 15 up any higher.;) There are a lot of times when I'll turn the 22 down to gain field of view, less boil etc. etc., but at looooong range the 22x sure can come in handy at times.
If it is a dedicated long range rig and the size won't matter then, if it was me, I'd go with the 5.5-22. I've been 100% happy with mine and wouldn't go smaller on my dedicated long range rig.
One thing to consider is that if you go with the 5.5-22 and the NP-R1 reticle, you will have the 1moa increments at 22x but if you drop it to 11x you will have the 2moa increments which is the same as the NP-R2 reticle. It's also a given that you should check this with your own scope but mine has been spot on.
Also, if it was me, even though there are several vendors around here that sell them, I check with Bill and Steve at: Link--> Shooting Supply - Buy Guns and Optics Online at our firearms store.
They're great guys with great service and prices. They have been around here a long time and a lot of the guys on LRH are 100% happy with their dealings with Bill and Steve.
Good luck with whichever you choose and let us know how it comes out.