Just wanted to garner opinion on what kind of Planets i should look to colonize for my next 3-5 planets or more.

I'm currently Rank 66, and my current intake is;

932 research per hour from 6 Planets.
750 artifacts per hour from 5 planets.
478 minerals per hour from 5 planets.

As of right now i'm maxing out my research cap every 9-10 hours, so i don't think i should increase that, even though i could really use the extra research right now, as i really want to get better planetary weapons/shields.

I'm sort of unsure about more mineral or artifact planets given that my upkeep is relatively low right now(523K), which i earn easily every 2 hours, and i'm running low on cargo space from all the other good artifacts i've got saved up and don't really want to scrap(214/285 Cargo Space Used).

Also i wanted to know whether or not its worth it to upgrade from +4 structures to +5 structures, or should i wait until i can build +6 structures.

More artifacts means more chance of Rescued Prisoners, Ship Bots and Tesseract Containers - which can give you more space to store excess junk

If you are already hitting your caps fairly quickly, then wait until you can build +6 (unless you are over-flowing with spare cash and don't care about dumping a huge amount of cash into new structures for a fairly small return - if you DO have more cash than you can possibily use at the moment and wouldn't miss the building cost of the +5 structures, then you might as well go for it)