Changes are as follow in 2015
FS pleated filter with foam from FX Donaldson style filter
22" tires vs. 20's
Wide smooth vs. skinny ribbed front tires
4" deep deck vs. 3 1/2"
Central located gas tank vs. dual saddle tanks
Electrical fuel gauge vs. float style
More plastic
better padded seat vs. old vinyl seat
solid mount seat vs. hinged flip up seat
battery located in rear fender vs. under seat
That's really the biggest differences, can't remember if they went to 3400 ZT's but think they did due to tire size.
As far as which one is better, I'm waiting to see how the 2015 holds up but the old school Fastrak is bullet proof. I have the 54" with an FS engine ( standard not SD fastrak) that just hit 659 hrs. today. I've replaced the deck belt twice, the hydro belt once, the center spindle bearings twice ( outside spindle are untouched), tensioner pulley once, fuel valve once, throttle cable once and serviced the hydros twice. My mower probably eats close to 25 acres every two weeks for the past 3 years and sits on an open trailer year round rain or shine. You could buy a new one cheaper than I would let this one go for. But like I said don't know what the new ones are going to do, hopefully they will turn out to be just as good as the older model but time will tell, if you do go for the older model you won't be disappointed. Not trying to push ya one way or another, rather just stating the facts that I can stand by.