The only comment I'd make is in some installations the labour to install can be a big cost so the overall difference in cost between new and used might not be as much as you might imagine it to be. To look at it another way the second hand price needs to be very cheap to make it worthwhile in my opinion.
The Sandels are great products, the 3308 is getting a little long in the tooth but still a good unit.
I'd seriously consider an Aspen, being a newer design it will have a greater degree of future proofing. They interface with just about any piece of Avionics you can name, much like the Sandel, unlike the Garmin equivalents. Also Aspen are introducing more and more functionality all the time like ADSB etc
With the Aspen you also get an EADI, an altitude alerter, minimums alerter and flight director if your auto pilot supports a flight director. There's also and airspeed tape altitude tape, VSI and rate of turn and balance ball. You can replace the whole 6 pack with an Aspen if you so wish.
Last edited by 27/09; 13th Apr 2013 at 08:52.