I've owned all four of those rifles, at one time or another - multiple Ruger 77 RSI's, a 7x57 #1 RSI, a CZ-550, and a .358 BLR.
Of those four rifles, I currently only own the #1 RSI.
I already have a 6.5x55 - a Model 70 Classic Featherweight - and prefer it to both the M-77 & the M-550, for it's superior accuracy & trigger to the others.
As much as I like the 6.5x55, the 7x57 is a marginally better cartridge - but either is an excellent choice.
The 7x57 is certainly more viable, in both the wide availability of factory loads & handloadable bullets, than is the 6.5.
That means, in a pinch, when you're off somewhere and in dire need of more ammo - you're more likely to find some 7x57 than 6.5, unless you're hunting Scandanavia !
The BLR is too much of a clash of styles, for my tastes - but YMMV, of course.