In Fallout 3-
Above Craterside supply is the cockpit of one of the planes that were used to contruct the town. It appears to be a present-day Boeing/Airbus airliner.

http://images.wikia.com/fallout/images/7/78/Craterside_Supply.jpg

In Fallout: New Vegas-
In Camp McCarran, the planes appear to resemble DC-3s with Folding wings.

http://images.wikia.com/fallout/images/7/73/Transport_plane_02.png

Which is more faithful to the series?

Doen't matter. There are jet planes and propeller planes today. Don't see why having both types in the Fallout Universe as a problem or one.

Isn't the plane at megaton supposed to be a military bomber? Hence the undetonated fat-man bomb?

I don't see any issue with both types of planes appearing the series...
They are on opposite sides of the country, and it isn't like we don't use multiple brands and models of jetliners today...

The good old trusty douglas dakota transport plane was still being used in Australia in the 1970's, prop planes are still being used as commercial passenger transports and are very common, the flying doctor here are prop planes, in vietnam they went back to some prop planes because they were better suited to roles that jets werent such as bombing and recon, the hercules which has basically been the staple transport plane for many years is a prop plane, The bomb in megaton was an undetonated bomb that was dropped during the war, thats why its always the question why the hell would you build a town around the crater of an unexploded nuclear bomb, supposed they wanted to be the first on their block with a nuke in their backyard.

I'd go with NV being more faithful to the series - it comes down to 50's theming.

In either case, I doubt any of those planes had flown for years before the bombs dropped given the price of fuel.

No the plane parts at Megaton were scavenged from a location north of Megaton which is now long gone

they could be modeled after any real world planes. doesnt necessarily make them run on the same fuel supply as the real world conterparts or make one more more faithful to the series than the other.