The Getrag isn't that bad a transmission if you take care of it. Make sure to overfill it if you can and don't drive it like it's a racing trannie.
If you're set on the swap go with the NV4500. It has a larger input shaft and is a stronger transmission overall. A friend that has a '99 with the 6-speed hates it. He said that every time he shifts into the split gear (I think it's 3rd or 4th) it's like he drops anchor.
I know one member here who did a swap to the NV4500, so maybe Mark will chime in with his experiences. But if it were me, I'd take the money and spend it on power mods and things of that nature and stick with the 'Rag.