My 550 FS is in .270 Win, because that's what I like. I have several .278-.280" diameter bullet moulds, and this package with its short barrel is just the ticket for launching lead under 2,400 fps. I would also like a FS in 9.3x62...just because. The 6.5x55 would be stellar in this configuration.
Get what you like! Personally, the .308 and .30-06 don't "do anything" for me. That doesn't mean they won't be work for you. If you are planning on hunting with it, game wouldn't be able to tell the difference between the two. My suggestion would be the .308, as it will perform a little better with the 20" barrel than the .30-06 will.