If your gonna level in niff you might want a bapho wep. That way you can mob and level faster. Maybe a +10 DDBP MG? But go for the bapho first since you already have dopple cards. I'm thinking this would be a good weapon to use with other classes as well as your Sin.
If you get a bapho wep you should be fine in niff, but you could also mob in better exp areas. (Check RMS)