Small Guns first and Energy weapons later, as always
You can tag energy weapons later on if you want, but by the time the decent energy weapons actually come along, you should be able to effectively put points in there without losing out on other stuff anyway.
Really, if you want to get the most out of Fallout 3 you need to get some kind of mod for it since they all add guns and stuff (As in, one of the larger overhaul mods that changes pretty much everything as well as adding a load of stuff to the game, there's no point getting littler mods unless they add some mechanic or something, since just getting a couple of guns but having the same baddies and quests is still repetetive)
Really, if you want to get the most out of Fallout 3 you need to get some kind of mod for it since they all add guns and stuff (As in, one of the larger overhaul mods that changes pretty much everything as well as adding a load of stuff to the game, there's no point getting littler mods unless they add some mechanic or something, since just getting a couple of guns but having the same baddies and quests is still repetetive)