They appear to be approximately the same in terms of performance.

Model vCPU Mem (GiB) SSD Storage (GB)
m3.medium 1 3.75 1 x 4 

Model vCPU CPU Credits / hour Mem (GiB) Storage
t2.medium 2 24 4 EBS-Only

t2.medium allows for burst-able performance whereas m3.medium doesn't. t2.medium even has more vCPU (1 vs 2) and memory (3.75 vs 4) than the m3.medium. The only performance gain is the SSD w/a m3.medium, which I recognize could be significant if I'm doing heavy I/O.

Would this be the only scenario where I would choose an m3.medium over a t2.medium?

I'd like to run a web server that gets 20-30k hits a month so I suspect either is okay for my needs, but what's the better option?