Likely all you need to fix the slipping gears is a new freewheel and chain. You're looking at $30 for parts and I would guess about the same for labor (if you aren't equipped to do it yourself). If that's the only problem, fixing the Raleigh is definitely more cost effective.

The Fantom CX will work fine as a road bike (with appropriate tires), but the ride will be more harsh than the Raleigh. Running wider tires (28+) will help some, but comfort was just not a priority in that frame design.

Also note that since the Fantom CX was a "some assembly required" bike, there's no telling how well it was assembled by the original owner. If I bought a used BD bike, I'd feel compelled to tear the whole thing down to bare frame and build it back up to my standards.