Vietnam and ToD are so very similar it's hard to recommend one over the other. Although Vietnam was the first SB game to be published and ToD the second, since they came out each has been updated in line with the other SB games so now the only real difference is the number of scenarios.
My feeling is that ToD is more popular than Vietnam. But I don't really know that should be so, I suspect it is still seen as the "newer" title and therefore must be better and, for the reasons stated above, I don't think that is actually the case.
My choice? ToD. ...but even as a veteran player of the game and the whole series I couldn't give you a rational reason why. It's just my gut preference. Whichever you choose you will have plenty of choice for opponents for pbem and you'll love the game.
..and let's face it.. you'll end up buying both anyway