My advice would be, assuming the fact that 7x57 is classed as a "military calibre" is an NON-ISSUE, is choose the rifle that:
1) Pleases you most in terms of features, style, and ease of use and/or magazine capacity
2) Barrel length. A 24" 7x57 is probably as good as any 20" 7x64 and a lot less unpleasant to shoot
3) Cost
The better round is undoubtedbly the 7x64 as it is more powerful AND MOST IMPORTANT on the continent now probably more easily sourced as loaded ammunition in various weights and bullet types than is 7x57.
That for me would be the major factor. That I could get factory ammunition and in the more useful types of bullets used for wild boar. Indeed in that respect the 7x64 is more versatile than the 270 Winchester...bullet selection in factory ammunition. It retains, does 7x64, that same advantage over 7x57.
The other factors would be if I liked the rifle or didn't. I like wood and blue steel and five rounds in a magazine. Three rounds, stainless metalwork and "plastic fantastic" just doesn't appeal to me.
Reject ANY notion that somehow you'll get cheap military surplus 7x57 with FMJ bullets for practice. I think that those days of modern made cheap factory ammunition in that calibre are long past!
Simply put in like for like barrel lengths the 7x64 is the better round on all accounts and is, in this 21st Century, the more widely distributed and easily obtained in various bullet weights and types. Whilst a good cartridge 7x57 is now very much "yesterday's cartridge" in terms of the likelihood of manufacturers making batches of it with their new bonded, partioned, TIG, TUG or etc., etc., bullets.
The 7x57 with a 173 grain bullet at 2,450fps is a true "killer" (it is what the 303 Mk VII was a copy of) BUT it may now only exist in old fashioned "cup and core" bullet styles. But out to 100 yards it is an efficient "killer" have no doubt but it is with the lighter 140 grain and 150 grain loadings that the 7x64 wins out as it gives a better round for shooting out to two hundred yards due to its higher muzzle velocity.
If you handload and use 173 grain bullets then for all practical purposes at under 100 yards there is no difference and beyond that?
Maybe a difference between a "point blank" at 150 yards and good to 175 yards (7x57) and a "point blank" at 185 yards and good to 225 yards (with 7x64) with those lighter 140 or 150 grain bullets?