My order of preference is pretty much the opposite of the graph - for these reasons:
1) I usually feel I get more for my money when I buy the requested game types (RPGs, adventure games, shooters) - so I don't mind paying out for these anyway.
2) Most RPGs & adventure games have little replay value so I tend to trade these in when I'm done and get some of my money back anyway.
3) The requested game types tend to be the genres that I want day one or shortly after release - so these aren't the type of games I want to wait around to come on PS+.
4) The requested games types tend to be the bigger games that fill up a lot of HDD space.
5) The non-requested game types (puzzle, multiplayer, sports, fighting, racing) are perfect choices to be downloaded digitally via PS+ as they're good to have conveniently on the HDD due to the "pick up & play" nature of these game types, they tend to have better replayability (you wouldn't want to trade them in as much), they're game types that you might not have thought about buying in some cases - so PS+ is the perfect opportunity to try without having to buy.