T3 are intel, newer than the T2, and perform worse than the T2. T3a are AMD and perform on par/slightly better than T3 for less cost. (From my own testing. Not a claim I can backup this is just my observation)

I thought they were cheaper due to the better energy efficiency. Less electricity means less cooling required, double whammy!

Where did you hear that? We're running a handful of m5a instances with fantastic performance. I figured they were priced lower because they're cheaper to purchase and operate.