I'd go with neither of them, to be perfectly honest! Diamond and Pearl really didn't do a good job as far as bring trype variation. In Sinnoh dex, the only fire Pokemon are Ponyta and Rapidash, and the only electric types are the Pikachu and Shinx evolution chains.
Neither option in Sinnohdex is preferable, or neccesary, as many of the obastacles that bend to electricity can be avoided by special sweepers such as Alakazam. Personally, I'd take the slot you're using for your electric pokemon and replace it with a more versatile pokemon, like one in the Starly evolution chain. I've had my level 68 Staraptor since I caught her at level 2 when I was only ten minutes into the game, and not only does she one-hit KO all of Aaron's pokemon, but she can throw down with most of the other elite four members' pokemon as well, but I digress.
So, to sum it all up (sorry for making this so long!), wait until you get the national dex to put an electric pokemon in your squad. And, if you'd rather not wait until nationaldex pokemon, then go for Luxray simply for two reasons: 1) His base stats are MUCH better, which gives him the potential to be a physical sweeper with the right moves, and 2)Luxray can learn Dark-type attacks, while raichu can't. This "Electric-Dark" combination of attacks can tackle a much more versatile spectrum of opponents than Raichu's Electric-only moves.
Just my two cents, again I'm very very sorry for making it so long!