Meh i have both, they both arn't that special. Im probably just undermining L4d2 because of how ignorent and bastards Valve were. L4d2 should have been a expansion/Paid Dlc not a stand alone game. They have just took the game, added several improvements, a few new infected, 4 new survivors, combined it in some campaigns, threw in some player suggestions and called it a new game.
What they have added:
Melee Weapons & Wider range of guns on l4d's poor choice of weapons.
5 New Campaigns.
4 New Surviors (Not as good as l4d).
3 New Infected.
Sub Infected Special Zombies.
Zombies can be gibbed better.
Some new items
Weapons are not in piles anymore, obvious failure of them in the first game.
Witches get up and walk around.
Melee knockbacks isn't so fast.
Bad Character Design/Voicing/Acting
Bad quality music
Daylight doesn't suit a zombie game
Levels appear very linear, fake doors & big open boring spaces
Borderlands is interesting enough but alot of things do my head in on the game, the fact that it's level based disables you from killing enermies greater than yours, turning it into a real borefest quickly. The graphics are nice, pretty fresh, and the layouts are pretty good. But you really don't have any inpact on the world, kill a whole hive of Scrags, and they spawn in 10 minutes, complete a quest and nothing changes.
The conclusion to this is, wait until both games go down in price, to the extent where your happy to pay. Left 4 Dead should have got most of those improvements as DLC, and not on a new standalone game, i feel cheated as much as anyone else, they promised DLC for left for dead, and they gave us a boring campaign and pointless mode and said, that's it folks, now go buy l4d2.
Valve have lost alot of respect for this move, and left for dead 2 isn't even that great, they took some idea's we had been throwing at them for along time and said "Look mommie, i dids it, now wear it" A kid to the outside world, a childish move. Borderlands is really for anyone who can stomach full on Wow-like mmorpg's which is in single player. It's like fallout 3 watered down to the bare theme and then wow merged into it. If your buying borderlands because of the vehicles, don't they suck, the weapons are good though.