Manual. The 6 speed is just about unbreakable. you can really thrash it and it will take it.

The shift feel is really nice, not notchy or sluggish, so even daily driving is a pleasure with it.

I've never been a fan of automatic transmissions in anything but heavy vehicles but I believed some of the hype about this new technology etc. etc. and tried both auto and manual GT spec B. I was sadly disapointed with the auto, really not a lot has changed apart from gimicky paddles on the steering wheel. It was easily confused between 2 gears and always held gears too long under full throttle condition. I wanted to be able to open the throttle fully and pull the next gear and use the torque of the 2.5 but it was impossible. The trans would simply stay in the lower gear and hit the revlimiter regardless of paddle flapping etc. It often seemed to know better than what I wanted.
I like to accelerate without making a revvy fuss sometimes hence I went for the excellent 6MT. I also belive the manual wil offer better resale. If you're looking for a used TB STi Lib, I'd advise against the older 5MT version though.
^^ The above is based on my experience test driving 2008 GT spec B LIberty not the 'tuned by' , there may be differences in the trans mapping that I'm unaware of so treat this as advice only.