The combat in Fallout can be merciless, but its all about the first couple of characters. You can quickly learn what works and what doesn't. Melee for example is a big no-no, but your not going to get far using guns unless you have a nice Small Guns skill and Repair skill value. And get the Scrounger Perk and ammo problems become very few and far between. Apart from Mini-Nukes. Oh lord I wish I could get a stupidly large amount of Mini-Nukes without having to camp outside Flak and Shrapnel's for 3 game days. And the map system is fine
I found myself checking Borderland's map screen a lot more than I did on Fallout. [For actual guidance on where to go, not to figure out if I'm still heading in the right direction]
I've invested well over 300 hours into Fallout since it's release, so maybe my comments are biased. I also own Borderlands, I've completed it once and am on a 2nd Playthrough [merely for the level 50 achievement] I've also mucked about a few times online with a friend. Borderlands boss fights were so-so [I preferred the early bosses tbh] the landscape DID give me a very post-apocalyptica feel [even my father, who is a casual gamer, asked me if I was playing Fallout when I was actually on Borderlands] It's also quite a funny game, some of the characters have BRILLIANT accents and one-liners ["Lucky is my friend! And when I say friend, I mean asshole who busted up my momma's girly parts!" and I also occasionally find myself going "Come on everybody, Catch a riiiiiiide!"] and the little boss subtitles are worth a chuckle.
But I still find Fallout funnier. I dread to think how many creatures and humans I've seen slowly separate into several different pieces and fly off in other directions thanks to the wonder of V.A.T.S. I find the plot so much more gripping, there is a stupidly large amount of things to do even if you have finished the main plot. [Took my first character 95 hours to reach the point of 'Okay, done every quest and sub-quest and free-roam quest and achievement I can earn being an evil piece of work, time to roll a new one' 95 hours, baring in mind, after I'd already hit max level of 20 back when the game was still fresh, then went and did all the side quests, THEN the DLC was released with the increased level cap, so I dread to think how much lovely EXP I wasted]
I personally think Fallout3 looks a hell of a lot better, even if the world is the usual shades of grey and brown, but it LOOKS like an area that has been in complete decay since nuclear bombs fell 200 years previous. The main radio station in the game [that's right, radio, still alive and kicking in post-apocalyptica!] has some music that you really wouldn't think would work with the game, but strangely, is to this day one of the best soundtracks to a video game I've ever heard. Whereas Borderlands just has a kind of random background track. If Borderlands had at least given me the choice of listening to "No Rest for the Wicked" in game instead of just on the intro I'd have probably enjoyed it that slightly bit more.
Anyway this is turning into one long comparison.
I'd say go for Fallout if your looking for a truly post apocalyptic game with a single player campaign that should last you quite sometime [If you go looking for the extra quests and randomly like to explore new areas]
But if you are looking for multiplayer co-op chuckles, with a crazy large amount of guns [whom only slightly vary due to stats tbh], and enjoy getting zerg-rushed by large amounts of mobs [at least, they rush the fuck out of you in Single Player], get Borderlands.
