As usual in these threads, there are at the very least two kinds of people: Those that prefer FO3 to NV and those that prefer NV to FO3.
I think FO3 just has to be played, at least once, no questions asked. I think the DLC-added missions are ranging from pretty fun to absolutely hilarious.
I personally didn't like NV much because of certain content I just plain can't accept and because the whole thing ended up pissing me off on a hitherto unprecedented level, so I eventually took to just killing pretty much anyone and wiping out any and all members of all the big factions on sight; the game could not quite follow my hate-derailed train of thought, so it chose to ignore portions of my stance and (deplorable) actions.
Still, if you can get it cheap, don't mind gambling, have no problems being pretty much insta-killed by invasive species such as Mexican wasps of Certain Doom or fire-breathing Barnies, absolutely do get NV.
I just think that starting out with FO3 is the better choice. I love Fallout 1 & 2, but I actually liked the ride through 96% or so of FO3. NV pissed me off bad so I can't give any proper numbers on that one. If you play games for the fun bits, the weird bits and the action bits, both titles offer plenty of those. I just think that FO3 was the more diversified, more rounded package of the two. They both have - at times severe - reliability issues and might crash or quit to desktop badly. Just let your gut feeling guide you. Try to make sure to have at the very least 3-4 hours of uninterrupted play when you first start out - and do save often. Oh, and absolutely do use multiple save game slots, just in case the game derps out on you.
Also, FO3 saw me enthralled with exploration. Just walk through the desert, you might just find the most incredible things hard to describe or properly put into words that make sense to other people. NV, over large stretches, just made me want to hack a car into the game. Or a Harrier jump jet. Just to skip the tedious walking followed by way too much touchy feely talking. KILL THEM ALL! RHAAAAA! DEATH TO ALL OF THEM!
Hardcore mode might be enjoyable to you, it might also just plain kill you at the most inappropriate times possible. Your call.