Do folks have recommendations either way as to whether they
preferred netstumber for free versus the slight cost of wi-spy,
and any experiences with either?
You are talking about two different classes of products. Netstumbler is just a simple program to list AP's and peer to peer networking in an area. Fun, but not all that usefull for major troubleshooting.
Wi-Spy is a spectrum analyzer tool for 2.4GHz. From what I have seen it is neat for under $100, but lacks features that the nice laptop based spectrum analyzers (Cognio) provides. On a budget I think that Wi-Spy is a good buy if you need spectrum analysis.
I have the Wi-Spy device and it is quite good for the price. You get a decent idea as to what is happening in the 2.4GHz frequency band. They even have images of what a microwave oven should look like.
But as GT mentioned it is not in the same league as Cognio or others.
Get yourself a free copy of Omnipeek personal as well.