My vote is for the 6.5 X 55, but then I am a reloader.
Brass and bullets are easy enough to find by going online
to buy. ( Same goes for ammo. )
The 7 X 57 is a great caliber also though, you can't go wrong with
either one. Several of us have used the 6.5 X 55 for about 3 years
now here in the Texas Hill Country. It gets the job done without being
too much at the closer ranges. ( Like under 50 yards. )
Accuracy has been great, with groups under an inch @100 yards
being the norm. Of course, that's using reloads. Alliant Reloader 22 powder with 140 and / or 160 grain bullets being our preferred loads.