Ironically, I just got rid of my Nissan Sentra for a Mazda 3.
Both are good choices, my Sentra lasted me 12 years, and was still running when I sold it. I love my Mazda, but honestly I hardly ever use the manual part of the transmission, I just forget it's there where there's no clutch, so test drive them both, and pick the one that feels right. Either way, you're going to get a good car.