The Dell 1390 is just a basic B/G card. Not much to say there...
The Intel Wireless 3945 is apparently A, B, and G. You pretty much won't ever need A, but it's probably a better card than the Dell 1390.
And hotchevymanzach got it pretty well with the N card.
Unless there's a huge price difference between the Dell and Intel 3945, I'd go with the Intel card. Though you don't need A, it's probably a better card. Sometimes the budget cards can have pretty poor reception and what not.