Boy can I empathize with you (as can every mother that leaves their child in daycare). With my first, when I started looking at centers I also broke down bawling in the car. I eventually found a center I really liked and I had the ability to only leave him for about 4 hours for the first month or so because I worked from home with him the other hours. Those baby steps (no pun intended) helped me and him adjust. He was 6 months old. He did great, had a lot of fun with the other babies, and really socialized. He is now 3 years old and in different Montessori school and is doing awesome - he's never really known any different so even transitioning him to a new center was pretty easy. I now also have a 9 month old that I put into the Montessori school when he was 6 months old. It was way easier the 2nd time because I knew he would be fine, would be loved and would have fun. Plus, at 6 months old they sleep a lot - he still sleeps for at least half of the time he is there and then he spends time eating and then the rest playing, having circle time, doing "art", etc. There are days that if I think about it too hard it makes me sad so I just try not to think about it.
I think there are pros and cons to both options. The time between 6-12 months is soooo fun. They are more social, learning to crawl and walk, etc. So its an awesome time to hang out with your baby and really experience it. The downside is I do think it will be harder at 18 months because they are much more cognizant of being left than they are at 6 months. Maybe if you're able to ease him into it before going full time it would work. They are social little creatures and also they have to learn to get along with the others. I did find that when my first son transitioned into the toddler room at 18 months that is when things started getting a little more physical with the kids - sharing toys, pushing each other around, being out on the little playground, etc. For my son he was used to it already and he knew the kids - for someone just coming into that environment it might be a little intimidating at first.
But with all that said - the one thing I've learned as a mom is there is no right answer and you go with your gut and believe in your decision and don't look back. Your confidence and happiness will trickle down to your son. Kids are very adaptable and while if you wait until 18 months there might be a slightly rough transition period, he'll eventually do great.
Good luck!!