I strongly recomend you upgrade your existing blade. Simply cuz the stock blades that come with the FX sabers are NOT very durable, even after convertion is done. That said, you will probably not be able to duel with it any better than before. (in this case, you "only" get the brighter and removable blade, and the lighting system is moved in the hilt)
I recomend any of the Ultrasabers blades, simply becouse of durability. Mid grades are the "slimest" of the blades (still 1" diameter) but are MUTCH MUTCH better than any FX stock blade. It's like this: If you drop your FX bladed saber on the ground, you can probably say goodbye to it.
If you drop a Utrasaber, there is no chanse of that happening. They are made with dueling in mind so are perfect for that.
If you decide to go with a Ultrasaber's blade, i strongly recomend you go see the blade comparison thread where you can see diferent ways the blades light up the blade (Standard blades have thiner but brighter core, while Ultraedges have a fuller blade colour but with some colours can be a slightly pastel look).
Whatever you decide to do, you will get a brighter blade, regardles of what blade/colour you choose
Ultrasabers are just awesome