Depends a lot on the platform too. You do not want to play New Vegas on the PS3 for example ... sloppy programming makes the save-file unplayable when it reaches approximately 13 mbs in size. It also has many game breaking bugs that you may or may not avoid and several glitchy trophies especially on the DLC's. It's all about luck really and you pretty much have to finish in around 100 hours. Awful release that should haunt the companies involved for all time.
On PC I would choose both and start at the beginning with Fallout 3. It's not like you'll regret spending 200 hours scouring the wasteland because you later discover that new vegas seems even better.
On PC I would choose both and start at the beginning with Fallout 3. It's not like you'll regret spending 200 hours scouring the wasteland because you later discover that new vegas seems even better.