Well, I assume you by "in front" mean a 3.5mm stereo jack on the front of the HU, accessible when the HU is mounted in the car?

Definately go for a player with the RCA aux inputs in the rear - otherwise you'll have a cable dangling from behind the empeg, out through the dash and then connected to the other HU - ugly! Much better to hide all the cabling inside the dash...

As to 2 or 4channel AUX input (I can't remember seeing such a thing (4ch), not that I'm current on car stereos...) - what you lose with 2ch is the ability to use the empegs equalizer to affect front/rear separately and the use of the empegs fader. You'd have to use the other HU to fade.

/Michael