Soldering will cause minimal warping, but is not nearly as strong as welding. Personally I think soldering, and brazing looks ugly.
If you already have the Aluminum pot, I would go for weldless fittings.
If you buy from http://www.bargainfittings.com/
you will better off.
Their products look to be of high quality and everyone on this forum that uses them seems to be happy with them.
I personally don't use weldless because my kettles are stainless and I weld for a living. Otherwise that's the route I'd take.
Unless you know someone that is a very good welder you always take the risk of getting something you're not going to be happy with.
Who wants to pay good money for crappy service?