Hi. I have the Sharp IM-DR420. I would say definitely go for that over the N710. The N710 has only 1.2mW per channel @16 Ohm, whereas the Sharp has 8mW x 2 @32 Ohm, ie. basically the Sharp is MUCH LOUDER.
It (the Sharp) has also got a 1-bit digital amp and 4-pole headphones (it can also use 'normal' 3-poles) which gives it better stereo separation and clarity than the traditional one in the Sony.
Yes the N710 looks much nicer, but in this case it's style over substance. I've had quite a few Sony MD's, so I'm not anti-Sony or anything, but the N710 just fails to cut the mustard. It probably sounds all right though, just quiet.