The issue with both approaches is that neither really removes the fact that the ride is the main component.
If you use one track that everyone has to theme, then people are bound to dislike the track and probably not enter. However it's the fairest way of assessing scenery/support building skill and creativity. On the flip side, if you make people build and theme their own track then no-one will enter due to the massive amount of time it takes to build a full track like that.
I can't really see enough people entering (on this site anyway, we're mostly track builders and don't often do the massive themed projects), so if you want to set it up, by all means do, but don't be surprised if no-one enters.