Fact is this (half finished) game is glitched/bugged/lagged so badly its not worth getting.
That's odd. In however many hundred+ hours I've played, I have encountered exactly ten bugs. Half of them because I used mods and/or did something dumb (ie spawn 9999999999999999 suits of power armor in inventory for lulz) and the other half being easily corrected myself with the developer console.
Protip: Just because you're having trouble doesn't mean everyone is.
Anyway, as to the topic, I'm divided. Gabriel made a good point. Getting FO3 first would let you appreciate the enhancements that NV brings to the table. Playing NV would pretty much ruin your FO3 experience for you. I know it has for me. I tried to go back to FO3 and I just can't stomach it.
On the other hand, NV is pretty much like what FO3 should've been to begin with. Intelligent, witty, connected, tons of weapons, tons of
options. It also has a much firmer connection to the past games in the series. Whereas FO3 basically took some familiar faces and threw them together in a mishmash, NV has a much more plausible reason for all the various factions to be in the wasteland.
As Gizmo said, FO3's major strength is its map design and art style. Its roleplaying elements are subpar, and most of its mechanics aren't particularly well executed. If you do get FO3, you'll want to get the GOTY edition on the PC, as there are mods to fix a great many of the flaws of the game. Repair system svcks? Get a repair mod. Game too easy? Hallo thar increased increased spawns. FO3 without mods is pretty blah.
This advice also applies to New Vegas. The GECK for both games means that even if you have a problem with vanilla, you can probably download a fix for the issue. I was frustrated with how I couldn't craft specialized ammo at an ammo bench, so I downloaded a mod that let me do that. Indeed, there's even fixes for a lot of the bugs in the game, at least the ones that don't need an exe patch to fix.