If you plan on having more children, definitely consider a ranch home with a finished basement. I grew up in a huge colonial with high staircases, moved to a ranch when I married and now I live in a in a quad (4 floors) with my kids. The best thing about living in a multi-level house is PRIVACY. You can get away and have your own space on a separate floor from the kids providing they are older, but if the kids are small, it's very hard to keep an eye on them. The bad thing is that you have multiple floors to maintain. Cleaning windows on the top floor from the outside is a pain. Ranch homes are easier to maintain and if you have little kids, they are safer and it's easier to keep an eye on the kids. With a colonial, it's hard because you have to block off the stairs with gates when they are little. Also, if you find a colonial with a high staircase, that too can be dangerous with small children unless you keep them on the lower floor until they are old enough. The best type of colonial would be one where your laundry room is located on the same floor as your bedrooms and linen closets--that way you aren't carrying clothes baskets up and down stairs. Because if you are taking clothes up and down stairs with little ones around, you will have to keep shutting gates--that's a hassle, trust me. Overall, if you are planning on having more children, I would seriously consider a large ranch with a finished basement. From my experiences, if you have young children, ranch homes are the best--easier to maintain, less dangers. When your children are older, you could always move into a multi-level home.
Good luck in your decision,