I was on several AA RJs in April on a trip to Charleston, SC, so here's my perspective. Disclaimer, I live near BOS but flew ex-MHT since it was cheaper. And I'm 5'11, so you can't say I'm short. Not the tallest, but not short.
The E175 MHT-PHL was operated by Republic...meh. Average. Felt like an older UA 737 but the seats were leather.
CRJ-200 PHL-CHS operated by Air Wisconsin was better than expected, really dark cabin and dinky but it didn't make me feel claustrophobic, and there was enough room but just enough.
Mainline A319 CHS-CLT was the worst of the bunch. It was a refurbished ex-US aircraft but there wasn't nearly enough legroom. Slimline seats were super uncomfortable.
CRJ-900 CLT-MHT was very very nice, a nextgen operated by PSA. Cabin was very bright considering the windows were large. It was reminiscent of a 787 and had oodles of legroom.
So actually I'd much rather have a modern RJ than even a newer refurbished mainline 737/A320 family.