I like to balance the two.... For every appearance mod, I need something under the hood to match.
I'm not a fan of the primer gray with hail dents on Z28s, nor am I a fan of chameleon paint and awesome wheels on a Civic DX with the same old 1.6L SOHC 8V POS engine under the hood! There are certain mods I despise on any ride and they can be described with one word: RICE!
I had the following priorities with my car when I bought it:
1: Pass smog and safety checks (both passed without incident). Tune up engine and perform other maintenance.
2: Make sure the ride is comfy (add a nice sound system with some road noise control, condition leather, clean carpets, etc)
3: Make it FAST (PCM, plenum, intake, swaybar, etc)
4: Make it look cool: add more gadgets, fix that crack in the bumper, and add some HIDs, you know the drill.
Priorities three and four usually are done in lock step, it's more fun that way! Note..... three and four never seem to end..... I like it that way!