ST3 is noticeably faster for me with some plugins like SublimeIntel, and faster with large files as well. It may be my imagination, but RegExp "feels" faster compared to ST2 as well.

And as others mentioned, a few plugins are ST3 only. The linters are on my must-have list for editors. ST3 added support for additional "decoration" of the gutter and of code, as well as minimal "dialog" support - I suspect that's why those plugins are ST3 only.

If the syntax definitions for the languages you use are geared for it then ST3's symbol indexing (giving Goto Definition & Goto Symbol project-wide) is worth it alone.

I upgraded because of SublimeLinter plugins (inline error checking for js, jsx, ruby, php etc.) only available on ST3.

ST3 seems pretty stable for regular use.