the only difference is comfort and appearance.

Completely straight has a more aggressive/cooler look (in my opinion) and since it does not rise, your body is down that little bit lower and out of the wind a tiny bit.

Riser bars can look kinda dorky (again, in my opinion), but a lot of people wont' care. They will be a little easier to reach.

Various people will find either of those options more or less comfortable.