Slicks: Depends on what you're into. I kinda like going faster and quieter, myself. The constant vuvuzela-like hum of MTB tires on pavement annoys me.

Bar Ends: Right now you have one hand position. Bar ends give you 2-3 total. Is it a huge number? No, but it's 200-300% more than what a normal commuter/MTB has. Makes a difference on multi-hour rides.