I've flown the CRJ-900 several times SLC-IAH, which is about 100 miles further than LGA-RSW. Nice ride IMO, especially if you get Economy Comfort. Often one of the more likely upgrades to F also. My small Air Boss soft side carry-on will even fit in the overhead bin, although most rollaboards won't.

I would choose the 900 nonstop every time vs. connecting in ATL in summertime, especially if it would be an afternoon connection when the thunderstorms often get going.