I got a free year of WorldMate Gold and gave it a try, but it soon became apparent to me that it doesn't do the job that I want it to do. I kept track of all the many differences I found between TripIt and WorldMate, but to summarize, the primary reasons that WorldMate didn't work out for me centered around these issues:

1. TripIt does a better job of parsing the emailed itineraries from the travel providers that I use. WorldMate fails to parse ticket number, operating airline, price, and booking details on some occasions, whereas TripIt almost always parses the passenger name, ticket number, price, and booking details. This is probably a big YMMV, because I'm sure that WorldMate works better for others.
   
2. WorldMate places each flight segment into a separate flight entry, so that a PNR becomes a list of individual, unrelated flights that then must be edited separately. TripIt places each flight segment under the same flight entry, which facilitates managing the entry as a single PNR. This one was a biggie for me, especially when deleting/changing/managing flights.
   
3. On one occasion, Avianca sent multiple confirmation emails for each member of my travel party. WorldMate processed the first one but errored out on the second because the flights duplicated those from the first one, even though the name and ticket number were different. TripIt combined the separate itineraries into a single flight entry with the same confirmation number but with separate line items for passenger name and ticket number. It was then that I realized that keeping track of multiple travellers is something that WorldMate simply doesn't do. As I am often the organizer of the trips I take with others, this became a dealbreaker for me.
   
4. One small but significant thing that WorldMate doesn't do is store hotel check-in and check-out times. This omission took me by surprise, because I like to document the times that I plan to arrive and leave the hotel as part of my travel planning process. TripIt displays these events in chronological order as part of the travel plan for my arrival and departure days.
   
5. There are a couple of ways in which the WorldMate interface is superior: World Mate does a better job of popping up windows to facilitate the entry and selection of dates, times, and addresses, whereas TripIt often presents an empty box for me to type in with limited assistance.

Bear in mind that I am information-driven, so the busyness of the TripIt interface is something that I appreciate. It's certainly equally valid for someone to find WorldMate better for their purposes. Based on the above, I'd say that WorldMate is the simpler/cleaner of the two. My decision is made easier by the fact that the way that WorldMate organizes flights and hotels makes it unsuitable for the way that I organize my trips, but this might or might not apply to you.