Here is a link to some very good tutorial info about the different type of scopes and their advanatages and disadvantages:

http://www.astronomics.com/main/howto.asp?n1=1&t1=4&cat%5Fid=4

Things to consider in buying a scope include (IMHO):

1. Aperature - the larger the size the more light collected, the brighter the image.
2. Portability - Do you live in the city or country? The most important factor in what you see when viewing deep sky objects is the amount of light pollution. A smaller scope is more portable and easier to set up at a dark site. This is obviously a trade off between 1 and 2, but can also depend upon the type - Catadioptric is an option where you get more aperature for the total size.
3. Quality - of optics and steadyness of the mount.
4. Computerized or manual - if you are new to astronomy you may want to consider a GOTO scope, especially in the city where it is harder to star hop and find fainter objects.
5. Once you get the scope you will want to start looking at quality eyepieces which can greatly improve your views.

Just a few random thoughts. Email me if you have more questions. I have a Celestron Nexstar 5 which I love - great trade off of size and portability and ease of use.

-KQ