Depends on what site, your skill, your ideas, etc., etc., .

How long is a piece of string?

Twice the distance from the center.

I'm gonna actually answer your question.   Vectors.  When I look at the ports of top vector artists vs. Top photogs, usually it takes half or even way less vectors to make the same as it does with photos.  Also, there is way less competition.   Everyone with a camera thinks they can be a great photographer.  Most people can't draw well enough to be successful as a vector artist, and the difference is they know it.

All true. But vectors take longer to create, so you can't upload as many. I can take 100 Or even 1000 photos in a day (not that they'd all be worth uploading, of course), but five vectors is pushing it. And it's the same challenge as photography...finding a niche and creating quality images that are useful to buyers. Plus you really have to learn Illustrator and understand what the micros are looking for (closed shapes, expanded lines and text, etc. etc.) There are many Illustrator techniques that cannot be used for microstock.

There's more competition with photographs, but they also have a higher chance of getting extended licenses.

It doesn't matter if you want do submit photographs or vectors, you still have to be really good at it. At the end of the day, it's really dependent on your style and skillset.