Yeah simple answer get both, especially the GOTY or Ultimate editions of both. Neither of them are all that expensive. Then if you really enjoy them go ahead and play all the others, it certainly wouldn't hurt anything. --- "Oh dear! The guests will have to eat hamburgers for dinner tonight."
If you want the best overall Fallout experience, both story-wise and mechanically, spend an extra $5-15 and go back and start with Fallout 1, and then Fallout 2 (pretty much any computer will run them both nowadays) before moving on to New Vegas.
If you just wanna run around shooting stuff FPS-style in a post-apocolyptic-themed wasteland, play FO3. --- Forgive him father, for he knows not what ticket he purchases... Yeah I do... see? Zombies! =^.^=
New Vegas royally screwed me over with glitches a couple times. Once, I was about 3 hours into a mission, and I got stuck inside a rock. It crashed my Xbox. Once I rebooted it, I had to start the mission all over, and I never trusted my autosave again.
I eventually just sold it to a friend of mine, after I got myself in a rut that was nearly impossible to get out of, and was not worth starting the whole game over again. --- Go back to Kaipo, you spoony bard!