Either could be really cool, it wouldn't hurt to work on both a cover and an original and go with whatever is sounding best when it comes time to perform.

But since you're having trouble with solos and since it's tough to hold non-musician's attention with them, here's my advice:

Go with some kind of rhythmic break/breakdown type bridge, so you're still playing chords, but it's catchy and different from the rest of the song.

"Whirlwind" and "Second Skin" by The Gits and the bass part to the bridge of "Too Many Puppies" by Primus come to mind.