A 550 would be much better for highway riding/touring.
Ive had both my 350F and a 550K out on the freeway.
550 is much more stable, and you don't have to flog it to keep up with freeway speeds.
Properly tuned and maintained, it'll be right at home at 60-80mph.
The 350/400 bikes are great, dont get me wrong. Loads of fun, just better suited for the twisties IMO.