Good question. There are pros and cons to the situation. Generally, having some in-depth start-up experience is a plus no matter what... even with a 'failure', since you learn so much from it. (Though of course a successful start up may be even more of interest
, but if you can speak to what you learned from the experience it can still be valuable). The down side is that you don't want it to sound at any point like you are applying because you are at a loss/don't know what to do. In terms of when to apply though, that is more based on your personal preferences, and if you'd like to go for it now and think this is an ideal time (with 6 years of experience it may be), then you can just make that case in the application.
Good luck on your decisions!
mbaMission Insiders Guides: http://www.mbamission.com/guides.php?category=insiders
Free Consultation: http://www.mbamission.com/consult.php