Well I can talk first hand about a welded diff. I have one in my car and absolutely loathe it. I really do not recommend welding your diff if you drive your car more than one per week.
With the welded diff, making a turn into a parking spot say you were going to the grocery or the movies, one wheel fights the other as they are physically welded together as the outside wheel wants to travel more than the inside wheel. Hence the "chirp....chirp..", more like a "chung...chung....chung...."
This destroys tires too guys. If you strictly drag race or road race your car go for it, cheap and effective. But if you look at it from a practical stand point - I strongly do no recommend it.
I have had no problem in the wet, but once it does rain, I drive like a grandma, as the last thing I want to do is loose the car getting off a highway exit and have a new Armco hood ornament.
Regards - Yasin