I'd suggest the 5x5 personally, it takes less time (obviously) so it will be easier to get into the event, and then if you want to do 7x7 later you will find it more comfortable having already practiced 5x5. IMO, both are fun though, so I don't think there's a bad option.
Also, for those in disbelief of fast 3x3 without knowing 5x5, bear in mind that Maskow owns everyone at multiBLD and doesn't solve anything larger than a 3x3 (I don't think he either owns anything larger or knows how to solve them).