RE: 6.5x55 vs. 7x57
I can't comment on the 7x57 because I don't own one yet, although I wouold dearly love to add one to the battery.
I do have three 6.5x5 rifles. Two are Husqvarna M38 Mausers (one military and one dropped in a sporter stock) and one is a new CZ 550 American. All are fantastic shooters. The CZ 550, out of the box using some old PMC 139 gr PSP factory loads, printed several ~1 inch 5 shot groups @ 100 yards. I am still working up a handload for this rifle and hope to have it all tuned in for this fall's deer hunt. My biggest complaint about the CZ is that the action is not really all that smooth, and the trigger is just a tiny bit creepy. These are minor gripes; I still recommend this rifle.
The 6.5 Swede is one of my favorite cartridges for target shooting, especially with 140 grain bullets. The recoil is very mild and the inherent accuracy is great. I'm also quite fond of the 8x57 JS Mauser. I've got three of these as well, two military and one Remington 700 Classic. The 700 is being readied for a hog hunt soon.
These old calibers aren't flashy, and they are definitely calibers that you'll want to reload for to get the best performance in your rifle, but they've stood the test of time. They get the job done and they do it well.