I don't know anything about Waldorf.
Montessori - we have visited some and I have friends who send their kids to them. They are not for me, but I'm sure you'll get lots of replies from people who love their Montessori school. What works for one child/family may not for another. It is like asking opinions on different religions or different cities to live in.
IMO the pro is the cool toys they have. They really use some neat stuff. I also liked how they gave the kids "real" things to use - for example, a small pitcher of water to pour into small cups. That is a skill that kids need to learn and I liked that that was a choice during the free play time. They also seem to attract a fairly diverse group of students so that is always a plus to me.
IMO the cons were how strict they are with how the toys must be used. Bothered me in that kids that age are so imaginative and just exploring everything - seemed wrong to me to tell them "no, don't stack it that way - the right way to do it is like this" I think, let them try it different ways and come to realize that if you stack the bigger one on bottom that it can be built higher - and hey, they may find a different way that also works.
For my child, it didn't work because she is very social and was very upset that she wasn't allowed to play WITH her friend, when she had already asked him and he replied that she could. A teacher actually came RUNNING over with a bad look on her face, to get my child away from her friend and "his work". She was told she could use it after he finished and replaced it on the shelf. I understand that is how they run things - it just didn't sit right with my child or me. I have also noticed (and of course may not be true of all Montessori - just the ones I've been to) that they are not very warm and fuzzy. One of my prereqs for preschools is that the teacher loves my kid and shows it. I want a genuine smile for her when she walks in the door - I want her to be open to a hug goodbye from my child at the end of the day (my kids are huggers like their mom!) I just want a happy, fun, easygoing teacher. Now, of course there are non-happy, non-fun, non-easygoing teachers at EVERY style of school - in fact we do not continue into the 4s class at the co-op we go to when each dd is 3 because the 4s teacher just doesn't work for me. Friends who have gone through that class (and love her - everyone is different) say she loves them like a drill sergeant - whipping them into shape for kindergarten. They are FOUR - I don't think they quite need to be whipped into shape yet...
I think it depends on you and what you are looking for for your child. How many days a week/hours a day do you want? Are you looking for a place that focuses on socialization or academics? Is price a factor? Your child's personality also plays into it a lot.
I would look at a variety of types of preschools and see what you think. I have found that I very quickly get a gut feeling one way or another about each place I visit. I personally LOVE co-ops (often run through local community colleges so check their websites for info) where your 3 year old child goes two half days a week and you work one of those days. Really a nice transition from home with mom to school life (not to mention very reasonably priced). If you are looking for a more classic drop off preschool, check out NAEYC's website (National Association for the Education of Young Children) and see if they have any schools in your area that are accredited. That process is quite involved and some preschools that are really great just haven't gone through it but still apply developmentally appropriate procedures. If they mention DAP in their paperwork and play Tom Hunter during the open house, I know I'm in the right place!
Good luck making your decision. All schools are different and I hate to even make these comments because I know you will get replies from those who adore Montessori - and maybe I would too if I saw theirs. The ones I've seen are not for me - though hey, they may work great for you. I'm just giving you my opinion from what I've seen. Worth what you paid for it!