I usually buy individual papers, but sometimes I will buy a whole set if its a brand and a kit I LOVE. Here is what I do to decide:
-How many LOs do I really want this paper for? 1-2 or say 5+...how usuable are the items in it?
-Are there pieces I would NEVER use...if so can I swap them with a friend, or make a gift for someone else?
-Add up the cost of all the individual pieces from a set that you MUST have, if you can get everything you want for much less, I buy it separately. If its only a bit more and/or I can only get must haves from the set then I usually buy it.
I am a huge fan of mixing and matching brands on my projects. I swear half the fun for me is co-ordinating my own "kits" lol.