I am trying to work out if an RCD310 with MDI is a better option (and cheaper) than going for the RCD510 with a large SD card. can anyone advise on the following please

1) how does the sound quality of the two units compare ?
2) does the MDI work with a non-powered USB stick and what is the max capacity ?
3) does the rcd510 accept SDHC cards of 16GB ?
4) if i am not going to go with the reverse camera what is the advantage of the RCD510 other than the touch screen (i dont spend huge amounts of time in my car and often listen to the radio
5) how well does the navigation of folders/tracks work with the MDI on a rcd310 (i know it works really well on the 510 as i have an RNS510 in my current skoda.

I am trying to keep the cost of the options list down and dont want to over spec the car as you dont get it back when you sell it however i dont want to have to live with an inferior system for a few years. I am thinking maybe starting with the standard fit 310 (SE model) with MDI and then upgrading in the future to 510 from fleabay if i feel the need.

you advice and experience would be most welcomed.
cheers