I always like to dabble in both. My car, though new, has a couple exterior mods, but also a performance one. I like to keep things balanced like that.
My Grand Prix went balls to the wall performance first, then made her look better. She started off as a plain jane SE with a 3100. I got a junkyard engine/trans from a GTP, stripped it down to the block, put cam, valve springs, retainers in her, ported the blower and intake, headers, threw it all back together and dropped the pulley down 3.75", got tuning software to custom tune it myself, and a fuel pump rewire kit. Also, threw in some lowering springs with new control arms and struts/mounts all around and a 3" exhaust.
After I slapped that in her, I went with GT bumpers, SLP GTX spoiler, factory hi-po rims, and I still have side skirts laying around that need painting and installed. She also has black projector headlights and LED tail lights. I also wired in the digital DIC that the GTPs had, including a boost gauge. I hoped to put a HUD in her too, but I don't know if that will ever pan out.