Alright gentleman I've got a dilema about my future career choices, and I could use some outside views and opinions on a pretty big decision.

Quick cliff's om where I'm at right now

-Graduate College next December with a 4 year degree in Construction Management
-Offers from several global companies to work full-time after college
-Salaries ranging from 50k to 60k, standard benefits and 401k's
-Jobs would either be typical desk 9-5, or move around the country for 10 years following large-scale construction projects.

Now, where it gets difficult

-Friend of mine owns a semi-successful lawn and landscape business back home (~150k gross/year right now)
-I have worked with said Friend throughout high-school and some weekends during college as an hourly employee of his
-Friend has a great head on his shoulders, has great business ideas
-Wants me to become a 'partner' with him after College
-Financially, because he has all the initial investment int he company, it would be a 30/70 % profit split initially
-We would branch out into the Construction remodeling industry as a part of his company
-Branch into equipment buying/selling, possibly auction service
-As the company reaches ~250k gross, percentage would be 40/60
-When company reaches ~500k gross, percentage would be 50/50
-My title within company would be "COO", responsible for all daily operations and overseeing labor force

So, my dilema is this:

Accept a job offer from a stable construction company with benefits, OK pay (midwest living FTW), and consistent work; OR, take this offer from my friend, which involves much more risk, probably lower pay right off the bat but with the opportunity to potentially make a lot of money down the road and be involved in a growing business, work for MYSELF, not a typical 9-5 job behind a desk. I have always had an entrepreneurial spirit, I ran my own landscaping business in highschool too, and I run a small construction company here at school just to earn some income (no realistic chance of it growing and becoming full time in this area). My major concern about accepting my Friend's offer is that if it doesn't work out, will I be able to get back into the Construction Industry...or is my only chance to accept a job right out of college? There are obviously some more pro's/con's which I'm sure you guys can figure out.

So, any opinions are welcome on what YOU would do, or what you think the best decision would be. I know everyone is different when it comes to this kind of situation, so I am open to all opinions. Reps to legit insightful help. Thanks gent's