Then it's a no brainer. Start with your new employer, then apply to your friend's startup whenever they're ready. So you don't miss an opportunity with the big company. You can transfer your H1B as many times as you want as long as you have at least 6 months remaining on it. They will use premium processing, you can be done in 15 days. Then if you're into your first 3 years on your H1B, they will be able to extend it for 3 more years during the transfer.