I only played the demo of Ghost Recon, but I thought that GRs graphics "looked" better, but OFPs graphics engine was far more impressive (huge areas - if you can see it, you can go there. Lots of structures, buildings and trees and whatnot too). OFP also has lots of different vehicles that you can operate (tanks, cars, trucks, boats, planes, choppers).
OFP also has very open missions, you are given a mission briefing on how to accomplish a given mission, but you are normally allowed to do it however you want. For instance, there is a mission where you have to sneak into a base to blow up some enemy armor with satchel charges. Instead of using satchels, I snuck in, stole a Hind helicopter, and blew the crap out of the base (and anything else that moved in the area). Mission successful.
I also like how the weapons fire a bit realisticly. They aren't hitscan weapons (weapons where as soon as you shoot, the "bullets" reach the target), you have to lead your target (if it's moving) and adjust for bullet drop if the target is far away (having to adjust for wind would have been nice too, but oh well). Makes sniping very challenging. The weapons themselves are true to their real life counterparts, they are all realistic in its range and rate-of-fire (except for the LAW which is really a single shot weapon, but in the game it can be reloaded) and there is a nice assortment, with new weapons (and vehicles) being added fairly frequently. You can create custom missions with it too, and there are lots of single missions and entire campaigns available to freely download on the net created by fans of the game (and they are generally really small, typical single missions are usually well under 100k).
The only knocks against OFP I can think of is it might be a bit overwhelming for a new user, and the multiplayer netcode is terrible (they say they are fixing it in a new expansion pack coming July).
But like I said, I only played the demo of GR, I'm sure it has more to offer than the bit I saw... there are a few "Ghost Recon vs. Operation Flashpoint" reviews on the net...