atmega32u4 vs atmega328
ATMEGA32U4 has more features in almost every department, and should be able to do anything a 328 can. More IO, more ADC channels, more RAM, more timers, etc. Plus it has hardware USB, which is an awesome feature.

I'm kind of surprised Atmel didn't pimp the USB series more. I only discovered them in the last few years. Coupled with the free LUFA library they make things a snap.

Your question is a bit.. lacking, though. Echo what? Through what interface?