Love the Rain-X...works great.
The Prestone de-icer will remove more ice, because it has more methanol in it...it will also remove more rubber from your blades and wear them out quicker...
Depends what your needs are...
For me, Rain-X works best, even in Vermont, where there was lots of ice, I preferred the Rain-X because on a long drive, the Prestone caused chatter as the blades wore on the glass...I just carried an ice scraper and let the car idle 30 seconds while I scraped any ice. Ice gone, warm-up complete, hit the road! (I used to drive from Northern VT to NYC once or twice a week, 350 miles each way...)
Cheers,