Ok so by "mess around" do you mean anything that involves you leaving the ground and coming back down? If so, I would make sure to have as much Chromoly as possible in the frame. In anything that will be bearing weight as you land, you need that.
If by messing around you just mean riding to and from work/school, or to just hang on at the park, I guess Hi-Ten would be alright, but I really can't say that I would ever by anything where the frame is Hi-Ten...I would atl east try to get a Chromo front triangle
Of the 2 bikes mentioned...the GT sounds better because it is Chromo.
If you get a good Chromo frame, and then get more into doing tricks, you can just replace cheap parts with good ones. I always tell people to make sure that the skeleton of the bike (frame;fork;cranks and pedal axels) are full Chromo. Second would be double walled rims...
what is your budget?
I've Got sXe!!!
Up The Antix/Up The Punx
1988 Mongoose Caifornian Pro
2015 Surly Krampus
"Running Is dumb" - Dave Lawrence