deciding between nanny/preschool/Montessori

I have had my son with a Nanny since he was 4 months old as we both work. Now he is almost 2 and has a new Nanny. I think he ges very bored at home and adjusting with a new nanny is very hard for him too. I was wondering if you could help me decide if I should put him in a pre-school "primrose" which starts next week for him or wait it out and let him settle with the Nanny or if I should put him in a Montessori that will take him in June.

Since we both work it will have to be full time. I really feel he needs some mental stimulation...he looks depressed most of the time during the day these days :(

I say go for the Montessori. We started my DD at 18 months because I thought she was bored at home and it has been the best decision we have ever made.

Did hse go full day?