well i have the wipeout hd platinum and believe me there is no comparison. gt5 wins hands down.
red bull challenge's difficulty lies in the fact that u have to pull off two perfect laps in 3 different tracks. these tracks have really hard turns that u must take really fast. breaking too much or too little, touching a wall or going out of bounds means you are not getting gold.
beating zico is pretty much the only hard thing about wipeout hd. the track for this is wide with no hard turns. the only thing difficult about it is being able to barrel roll into the bridge towards the end of the lap.
it took me about 50 hours to get the platinum for wipeout hd. i have spent more than that in the red bull challenge and i never achieved 3 golds(2 golds, 1 silver) before giving up.