Cruiser vs SS/FG, depends upon how far you ride and how often ? Just around the neighborhood, the cruiser is comfortable and may be the weapon of choice for riding the bike more often. If you do 20+ miles, the SS/FG more often gets you there & back faster, so it'll definitely be a bike you prefer to ride more often. As for getting in shape, I think the mountain bike you already have will do what anyone you have or are considering to get will do. The myb is heavy and has the same rolling resistance and gearing a cruiser will have, so in that regard, pedaling 38 lbs of bike will build leg strength. The gearing will also get the cadence/rpm work you need that both a SS/FG & cruiser will do. I have 3 bikes, a vintage 12 spd road, a SS/FG and atb/mtb. The atb is more comfortable obviously, but it does everything the other 2 do in terms of training and fitness, only slower and longer. And by that, I've ridden the same 30+ mile loop with all 3 bikes, the atb keeps me out riding the same distance longer. My cadence can be identical in certain gears, so cardio workout is actually better on the atb, as I will ride the bike for about 10 minutes longer. As for leg strength, the 38 lbs of bike in a hard gear with fat knobbies with more rolling resistance, no doubt pushing all 3 bikes just as hard, the atb will get me into better shape.