Rock,
I went through much the same dilemma when I was looking for a UTV. I looked very hard at the Polaris Ranger 700 XP before deciding to wait on the JD XUV. I am planning to post a review of the XUV in upcoming weeks, once I have had some seat time (now, mine has about 6 hours). I can say the following: for build quality, durability and ability to safely and easily haul loads, the JD is the hands down winner. It is a much superior machine in these areas. It also has a superior four wheel drive system, made better by the suspension, which is really nice. The Polaris is the winner in speed. It will outrun an XUV by some distance. The diesel XUV has 25 hp, which would equate to about 30-32 hp in a gas engine to approach the same torque numbers. The gas XUV has 23 or so HP, much less than the Polaris' 30 hp (500cc) and 40 hp (700cc) engines. The JD has a superior drive system to the Polaris, so it makes up some of the gap in efficiency but not the whole amount. The Gator also weighes about 200-300# more than does the Polaris, which is nice for durability but not nice for screaming performance. The suspension pieces on the Gator are much more durable, but again they are heavier. I personally felt the increased durability and workmanship of the Gator was worth the compromise in speed, but not every one feels this way. As far as size, they are quite similar in physical size, so not much difference there. Hope this helps.
John M