If you like the Elder Scrolls games because of the hundreds of hours worth of exploring you can do, then you'll love Fallout 3. I personally really didn't like New Vegas, mostly because the exploration aspect was awful. New Vegas has better writing, better characters, better... just about everything, but what it fails at is a huge detriment (for me), and that's exploration. New Vegas would have been much better if it was in a linear world, and it wouldn't be that hard to do considering the story guides you in a very specific direction, using things like overpowered enemies in certain areas to stop you from going where you want to go. Fallout 3 doesn't have that. You can go to any corner of the map you want as soon as you get out of the Vault, and while it wont be easy, it will be doable, unlike in New Vegas.
Basically, if you thought the exploration in New Vegas was okay, you'll be blown away by the exploration in Fallout 3. The other key difference that's in Fallout 3's favor is the atmosphere, which is much more post-apocalyptic and is in my opinion much better. But you'll have to judge that for yourself.
If I were you, I'd get the GOTY edition, if you can. The Pitt and Point Lookout are really great (The Pitt has a great atmosphere and Point Lookout has great exploration), and Broken Steel raises the level cap and extends the story (though the increased level cap means you can basically max out your skills without even trying, which isn't a good thing imo). Operation Anchorage is pretty fun IMO though it's extremely linear, plus you get a bunch of great loot at the end, and Mothership Zeta is atrocious... but 4 out of 5 is still good.