There's really very little difference between the standard and pro versions.
Cosmetic: The pro looks nicer (all black) and includes some nice metal parts including the mode wheel, stick base toggles and coolie hat. The multi-function display is angled a bit more and its controls were changed. The safety guarded switch on the stick also changes color based on whether the guard is up or down. All this stuff is really just cosmetic.
Function: The X52's resistance cone is plastic while the Pro's is metal. This mostly solves the X52's problem of wear between the cone and base ring it rests on. In the standard 52 the plastic cone starts to wear right away and will eventually begin to develop a notch that causes it to stick. The Pro's metal cone doesn't do that so it stays smoother over time. Also, the single X52 resistance spring was changed to a double spring on the Pro to give a more progressive resistance the further you get from the centered position. I personally like the slightly stiffer feel of the Pro.
All that said, the Pro is still an X52. Saitek tends to use inexpensive (read cheap) switches and they didn't change them in the Pro. I've had several standard 52's and the big failure point for me was the coolie hat (POV); however, I've been using Track IR for quite a while so don't use the coolie hat for views, it's programmed for other functions and doesn't get the workout it used to. The biggest disappointment for me with the Pro is that it has the same sloppy stick center as does the standard 52. There is too much play between the resistance cup and metal stick shaft and too much slop in the gimbals. This means you can wiggle the stick about 1/2 inch total with no response at all. I had hoped that with the switch to the metal cone the Pro's stick would be better in this regard but it's not. So much for hope. Not working out well for the country, didn't work well for the X52 Pro either. Also, they changed the throttle detent design with the Pro and it's not better, it's worse. The detent broke on my Pro after one year as did the detent on a friend's Pro.
So, is the Pro worth the extra cost? I would say no unless you're really into looks. The Pro's much better looking but functionally it's pretty much the same. Keep the standard version's plastic cone lubricated with a little lithium grease and it'll stay nice and smooth. If you think the single spring is too light then you can just cut out a plastic washer to sit on top of the spring to stiffen it up a bit.