Many thanks as always for the beer Duane!
Yes, you can use the site settings from smtp even for a single site installation, though where it really comes in handy is in a multi site installation when you want to use different smtp settings per site. In a multi site installation where you want to use the same smtp settings in all sites it makes more sense to use user.config so you don't have to maintain the settings in each site.
In a single site installation it just comes down to whether you want to expose those settings to the admin user in Site Settings. I can imagine some cases with a tech savvy admin where you would and some where you might want to prevent a site admin from messing with those settings. It is more convenient to be able to set it in the web ui but then again once set it typically won't need to be changed frequently so it can be better to keep them tucked out of the way in user.config.
Best,
Joe