If you need a phone with that kind of screen size, go for it. Honestly, I have an iPad to use for web browsing needs.
It all depends on what you use the phone (primarily) for. Calling? Texting? Music? Videos? Web browsing? I'd say the Incredible is best for calling, music, and possibly texting (because the HTC keyboard continues to be better than vanilla Android, and the other competition). The X has a huge screen; it'll be great for videos and web browsing.
However, realize that the Incredible will have a higher pixel density than the X, meaning it should technically look better (along with AMOLED; read up on the Incredible AMOLED dilemma). Also, the Incredible is slated to get Froyo at the end of this month (confirmed by Verizon on Twitter).
And a huge thing is Sense UI. Do you like it? Do you love it? Do you hate it? Play with the phones before you buy it; see which one you like best. Both are very similar as far as under-the-hood tech specs, the difference is the screen.
I'm probably a little biased for the Incredible, though. But good luck!