If you are developing a small to medium sized Umbraco CMS site, avoid choosing SQL Server 2008 over 2005 unless it is absolutely necessary or you have extra money to burn. The simple reason is cost. In the UK and just about anywhere else, you are paying a much more to use SQL Server 2008. Two price listings for web hosting companies are below. You can shop around, but the price markup will generally be the same.
The first example is eukhost.com. They charge an extra £30 per year. This is about a 50% markup for a small to medium site using one of their cheaper packages.
The next example is webhostinguk.com They charge and extr £20 per year which is about a 40% extra premium.
If your doing a low-budget site, I would recommend using SQL Server 2005 and then upgrading the database later on when 2008 becomes cheaper.