I have owned both, in the short barrel antipersonnel version, (still have the Nova) and I have shot the 870 in the longer barrel version extensively. I've also shot the longer barrel Nova.
What will you be using this gun for? I'm guessing hunting since you showed the long barrel models.
The 870 has been in production since 1955 or so and is the most prolific pump action shotgun in history (six million plus and still selling strong). It has more parts, accessories, spare barrels and everything else out there than any other pump shotgun.
You can put 30,000 rounds through one in a year and probably nothing will break (if it did, I'd be surprised). The Wingmaster hunting model is extremely smooth cycling and handles well.
The Benelli Nova is the new kid on the block. The stock is literally wrapped around the reciever of the gun, so if the gun doesn't fit your body out of the box, they can use some fillers to make it a bit longer, but it's basically "as is" out of the box. The 870 you can add longer/shorter stocks etc with the removal of a bolt and about ten minutes.
The Nova is light, handy, essentially indestructible and a good hunting gun where you don't have to worry overmuch about abusing the finish (half of it's wrapped in plastic). Around here they're catching on for ducks and dove/quail hunting. The fore-arm does rattle a little bit but not enough to make much of it, I took mine as a backup for a friend pig hunting (they listen well) and I didn't spook anything into charging us.
The 870 is the basis for 80% or more of the combat pump-action custom shotguns in the world, and you can find gunsmiths who can work on one anywhere. It's what you find in the dictionary under "ultimate reliability"
If you're only going to get one gun and want it for multiple purposes, and want to hand it down to your great grandkids, the 870 would be it (I know a guy who's got a 1956-made model from his grandfather... finish is a bit worn, but the sucker still works good... it's taken it's fair share of quail).
The Nova is catching up fast in reputation but it's only been out about four or five years.
Edit: the only reason I sold my 870 was a friend needed a shotgun and I needed money...... I replaced it with the Nova out of curiousity. I like it, but I'd buy an 870 again in a heartbeat too.
