Another thing to look at when comparing just those two cartridges...
Sectional density of a particular bullet weight.
Given the same weight in both diameters, the smaller diameter bullet will have a higher sectional density. This translates into higher retained energy as the bullet travels downrange. Example: Given that fixed bullet weight, and an identical muzzle velocity, the bullet with the higher sectional density will have more energy downrange than the one with the lower SD. That better retention of energy (and velocity), and the lower drag of the smaller diameter bullet will give you a flatter trajectory.
All that fancy math aside, both are perfectly good cartridges for anything up to medium/large North American game like elk or moose at medium ranges.
I've taken many whitetail deer and one elk with my Swedes and I know it's a popular cartridge in the Scandinavian countries for similar size game. Over there, it's not considered an oddball since it's been in use in those countries for over a century.
For deer-size game, the 140gr is a good choice. For larger game, go with the 160gr and let it hammer just as hard as a .277, .284, or .308 diameter cartridge of the same velocity range.
I've owned one .308, one 7mm-08, and I still own three 6.5x55s.
Why do I have the Swedes but not the .308 or 7mm-08? Mainly because I like having a unique rifle (be it my Gustaf Mausers or my modern CZ550).
For some reason I fell in love with the .264 bore cartridges.
They're easy on the shoulder, are renowned for accuracy, and perform well on game....but some guys will say the same about their .308s too.
And they're both right...