The WL.com site only uploads to weather underground every 15-minutes and it doesn't upload the 10-minute wind gust data that WU wants so you would have no wind gust readings at WU if you use WL.com to send your data. I'm not certain that the WL.com site has the CWOP upload function working yet either.
Ambient's server is really a Meteohub server sold under their name. I'd hate to take business away from them, but if you are technically minded you can "roll" your own by buying the component parts more cheaply and a software license from Meteohub. See this site: http://www.meteohub.de/joomla/index.php
for more info on what the Meteohub server is cable of handling. Even if you buy from Ambient (and you do
get a 3-year warranty for that price!), you will still have to visit the Meteohub site for any software updates.
I have both the WLIP and the Meteohub. They will work together, but there are some annoying data output conflicts that you have to put up with/work around. If you are not at all interested in using WL software to keep your data records or analyze weather data, then you should use a serial logger with the Meteohub. I wanted to be able to download a data copy into WL (I like it) AND have the Meteohub handling WU and my website uploads.
Another reason to get the WLIP is that if you have multiple computers on your LAN network, you can access the data logger from any of them without one computer having to be on all the time.