GTX is 102 lbs heavier, that will pretty much chew threw the extra 25 HP.
Power to weight
GTX 5.46 lbs per pony
GTI 5.71 lbs per pony
So its about 1/4 per HP, which is like the difference between a full tank of gas and a half tank of gas.
I am not a 4tec guy so not all that familiar with how these skis are fitted, I know the GTX is a higher end model but it is 5 years older. I am willing to bet that the 2011 GTI has more bells and whistles like IBR, then the 2007 GTX.
In summery to me the fun factor will be about the same, GTI is 5 years new, GTI is probably better equipped, along with the trailer. All that for an extra $500..................I am going 2011 GTI all day long if my assumption are correct.
Just my opinion.