L.,
For us, we went the simple preschool route. We chose a program that was run by a church, so it incorporated some faith, but wasn't even OUR church, it was more the principles of how to treat others and general Christianity. Also, it was run concurrently with the local public school district's schedule, which means that if the local school is out on holiday or snow day, so is our preschool, so we don't have to pay for breaks and days off (very nice compared to some other day care or other private based programs we checked out). Finally, our program had longer days - 9:30am-1pm for all ages - this was a HUGE plus. Even the teachers commented that they get more done with the kids instead of hellos, reading circle, play, eat and go home. Ours had a lot of "curriculum" - letters, numbers, writing and "themes" focused on them for the 3-4 and 4-5 classes.
These were more important in our decision making. I know other friends who paid A LOT for their kid's preschool programs. Other than maybe getting a foreign language or computer programs that our school doesn't offer, I didn't see any difference. I understand the basic principles behind Montessori schools. I think this is simply put a personal decision.
We knew we were going to be putting our kids in public grade schools, so we didn't see the point in spending a ton of money on a private preschool if we weren't going to be continuing them in the same school's grade school program. Plus, truth be told, they're just so young still. I have no problem with the idea of a Montessori program, I just never understood why they were so darn expensive?
BTW - both my kids are at the top of their classes (at 6 and almost 5). The teachers have told us they are well ahead of their peers in academic areas, great listeners, and good leaders. I think in addition to sending them to a good preschool program, what has really been important is having high expectations and "learning" programs for them from a young age. We never really treated our kids like "they're too young" to have expectations put on them. I think as long as you keep your child active with other kids (swimming, gymnastics, etc.) and expect him/her to play games that secretly teach them letters, numbers, reading, etc. then they will naturally be great learners who are eager jump into new things.
Best of luck in your decision!
Sara