Here to echo what Lola said about doing what works best for you. The one minute limit is part of the challenge for sure, but if you are more experienced and find that limit so constricting that it hampers your creativity, if extending to 90 seconds inspires you to create within the other constricts of the challenge, I say give it a shot - the point is to get you creating and stretching and thinking and learning. What if you worked on a 60 second version and then a 90 second "director's cut"? That could be a really interesting compare/contrast exercise. If you HAD to trim 30 a third of your 90 seconds out - what would stay? What could go? Is it possible to make the same impact or does it completely change everything? If you were a professional and tasked to do a 60 second youtube spot for a client - or had to fit just 60 seconds in for a fanvid show at a con but you had 90 seconds of material, what would drive your choices?

And if all that thinking takes the fun out of it then just do the 90 seconds one and see what happens :D