I've always used Trillian, so I'm biased towards it. I've always liked the interfaces, and it does what I want it to do. I've used pidgin a couple times and it just wasn't the same. If you're happy with Pidgin, I can't really give a reason as to why Trillian is better, besides the shallow fanboy stuff.

The only problem I have with Trillian is the Basic/Pro thing. The things they lock you out of when you don't go Pro is kinda dumb, like MetaContacts (taking a bunch of people and putting them under one contact, like if you have screen names for the same person across multiple services) and animated emoticons. vOv