One of the nicest features of Meebo is that it can be used w/o any installation – if you’ve got access to the web you can use it. One of the nicest features of Trillian is that it also includes Google Talk in its stable. I just saw this video of the upcoming release (no date yet) of Trillian Astra in which the developer suggests Astra will have a web interface as well:
The other killer feature of Meebo for me is the MeeboMe widget, which allows anyone to chat instantly even w/o an account on any service. If Trillian Astra offered a web-embeddable widget AND allowed me to interface with all four of the big IM clients I’d probably switch. Personally I prefer to have a client on my desktop as well, so if Astra offers the choice of client or web, and if they offer a widget (I’ve seen no mention that they are though, unless this is it), I’d be pretty darn happy!