it all depends on whether you want more power in your bike or not. both the 500 and the 350 are capable highway cruisers. Of course, the TB500 gets much more power and torque than the TB350. Apart from the engine, both the bikes are more or less similar as far as weight and dimensions are concerned.
If you have the budget, then we suggest that you go for the TB500. It will be a little less on fuel efficiency but as you mentioned, your primary rides will be on highway only, then more power always helps.