There both very entry lv bikes id say spend the extra 200 300 dollers and get something good that will last and not brake down the track.
WTP is better than gt from my exprences so i recomend the arcade. but as you get better at rideing your goin to break a LOT of peaces on eather of these bikes so i recon save a bit more money and get somthing good.
bike brands i recomend: FIT, Colony, easten, WTP, United
a good bike to start on whould have to b an EASTEN they can b cheep and good trail digger i think is the lowest one and its fair good. my mates had a "jane" for mayb 6 months now and hasnt had to repace anything. his a good rider 2.
things to look 4 on ur bike whould b sealed berings, 25 9 grearing, full chromoly frame
have fun tell us what u end up getten =)