Found startup or finish PhD?
Ultimately this is a personal decision, but I'll offer a few things to consider. Depending on your field and your specific research, taking time out might be a large set-back, or not. The world of research won't stand still while you are away and you might get pre-empted by a delay that might wind up to be longer than you first plan for. 

Your advisor should also be able to give good advice on this as s/he likely knows the field and what are the prospects of returning smoothly. Your advisor might, also, be willing to wait for you, or not, so it is good to check. 

Finally, what do you really want to do with your life? If it is to be an academic, primarily, think harder about finishing. If it is to be an entrepreneur, then the degree may not mean as much to you as taking advantage of the new opportunity. The industry experience might have some, but not likely a lot, of advantage in seeking an academic position later. Not a "slam dunk", anyway. 

Finally, make sure that you understand what the conditions would be if you join up with these investors. Are you a full partner, or are you being used? Either is possible in the somewhat messy real world.