If you're in it for fitness, just ride whatever you feel like that day. Cycling more frequently is better for your health than stressing about which bike to ride. Most of us have several bikes we ride and as long as your fit on all of them us comfortable, all is good.
If it were me between those two, I'd be on the Fuji because I'm a drop bar fan and not really comfortable on flat bars, but that's just me and YMMV.
Just listen to the all time best cyclist Eddy Merckx: "Ride as much or as little, or as long or as short as you feel. But ride."