First you should look at other threads on this board and other boards that ask the same exact question for a lot of opinions.
http://boards.ancestry.com/topics.software.general/2311/mb.a...For me I think that if all you have is an online tree you run the risk of loosing your data if the company goes belly up. You data may be less secure on the internet.
Most online trees don't have very good reports so if you want paper or books you may not get them online.
If you have stuff online, only put people that are 100 years old or older. Some people that are alive may not want their data placed on someones database where potentially anyone could see it.