Kismac is good because you can do a passive search. As in your computer just accepts data from wireless access points and laptops, and does not broadcast that you are in the area. So you can stay hidden from everyone while picking up everyones traffic. Then there are some nice tools in helping with cracking encryption keys that come with Kismac.
MacStumbler is more of just find and access point kind of tool, while Kismac is really for the cracker and network junky.