This is one of those questions that comes up from time to time. Some searches would most likely shed some light on the subject.
Personally, I've used both over the years...though the vast majority of the time I use purchased content. However, there are some paysites (solo girl and such)...where one is compelled to use sponsor content.
Purchasing your own content does allow more freedom of use when choosing who to promote. Also, if a sponsor completely tanks and you choose to change links, you can do so with purchased content. Obviously, if you're using sponsor content...changing links would be a no no.
Also, sponsor content frequently has watermarks...sometimes those watermarks can be downright ugly and intrusive on the pic quality. This is not always the case though as some sponsors are smart enough to keep the watermarks reasonable.
Lastly, there are some (not a lot), but some link sites that flatly decline sponsor content period. Most, myself included...will in fact list sponsor content provided the watermarks aren't ridiculous and the content isn't over-exposed.
So, in a nutshell...if you're going to use sponsor content;
Grab it asap when it becomes available to reduce chances of the scenes being recently listed.
Try to promote sites that aren't already heavily promoted.
Definitely read the rules where you're submitting to make sure they don't flatly refuse all sponsor content.
Good luck
