For me it's something like this:
-PS4 over Xbone, since I only have a PS4.
-PS3 over 360, unless it's an FPS, then I'll get it for the 360 if it's the same price or cheaper.
-The Wii is at the bottom of the list for multiplatform titles in the 7th generation because the Wii version tends to add some stupid gimmicky control scheme. I tend to stick to only buying exclusives when it comes to Wii games.
-PS2 over GCN, only because PS2 games tend to be cheaper. That being said, I will gladly buy the GameCube version if it's cheaper or close to the same price. I still love my GameCube.
-PS1 over N64, if I actually regularly bought games for these systems. Right now I only have one game for each.
-PS Vita over 3DS, though there aren't too many games that it would apply to.
Conception II is the only one I can think of off the top of my head, and I don't even have that yet.
-PSP over DS, like with the current gen of handhelds, this rarely applies.