I prefer Kopete. Pidgin is on decline because Gnome has switched to Empathy.
It will stay around, but it won't be the default IM client so will likely
decline in usage. This is not to say that it isn't good. The project has be
fraught with splits and poor decisions that have caused some users and
developers, especially the folks at Gnome, to give up on it. Kopete is KDE,
but it works in Gnome.

Install them both and find which you prefer. I find Empathy difficult to
like, but they say that it will continue to improve. Like Pidgin it is ugly
in comparison to Kopete which uses Qt4 and has and smooth and sophisticated
look. I am biased though and find Gnome in general ragged looking compared
to KDE 4.2.