45-50 lbs! holy cow. Even for a MTB that's heavy...
+ 1 on the older roadie although that cayne looks sweet!
As for pavement friendly ... You can go basic with some slicks $40-$80 and swap out your tires when you need to... you could also consider a new wheelset, like a light cross country wheel. A suspension corrected rigid fork, but in the end you may find yourself limited by gearing options...
I'd start with a pair of slicks and see if you're happy with it ... if you want faster, then go for a road bike ... or consider changing your gearing on the MTB...
the MTB will be a little bit more bomb proof for jumping curbs or riding down steps, but you'll definitely notice a difference, in speed, comfort and efficiency, even if you buy an older schwinn.