I find it difficult to decide. Now, I love GTalk, but if I were allowed to talk to my WLM friends (ie the 200 WLM friends vs the ..5 GTalk)via GTalk I'd do it. The windows are clean and tidy and they all tab nicely. WLM only gets to the same "place" once you strip away all the menus, and even then it's not my favourite. Then again, WLM is nice, and it's packed with features, and maybe that's its problem? Too many features packed. GTalks minimalistic UI has the VOIP, and the file transfer, and I've never found a *need* for webcam.
So GTalk + WLM friend list. Plus, GTalk works on my Blackberry (its pretty much a straight port, apart from VOIP), which is really useful. WLM refuses to, unless I use the dreadful online service which refreshes every time I send a message

Painful on a mobile device.