Do both.<p>Most of the time you can spare an hour or two every day to work on your 'big' idea. Spend it on research, try to find out if it has already been done and if someone else is executing right now.<p>99% of the time your 'oh shit, this could be huge!" of today will turn to, "oh right, that's why they failed" or "oh, great I'll never match that" or "hm, this is a lot harder than I thought", or maybe even "wow, that was really stupid".<p>If after three months of trying to shoot holes in it it still looks like a winner then it's time to start worrying about priorities.