In App upgrades vs separate paid version of app

[pressstart]

Since WP7, some apps were available in 2 flavors, paid and free, ad supported versions. I've seen at least 2 WP8 apps that have in app upgrades that would remove ads and/or add extra features. Case in point, Photostream and Battery (one of them). I assume it's a new thing for WP8, leveraging in app purchases? I still see apps that are offered in 2 versions. Are they there for WP7 compatibility? Why would devs choose to do the previous rather than the latter?

I guess my concern is, if I decide to do the in app upgrade, will it stick to my account like app purchases? Just wanted to make sure.