747b0392__5_1___Postgres_OnLine_Journal__Microsoft_SQL_Server_2008
[Feature] OGC Spatial Support - for the My dad is better than your dad fight in the GIS world between SQL Server and PostgreSQL/PostGIS check out A look at PostgreSQL and ArcSDE, Also check out our companion critque of the 3 spatial offerings [MySQL 5.1] Yes - MBR mostly and spatial indexes only work under MyISAM. Limited spatial functions. Some commercial (MapDotNet, Manifold.net), Open source GIS tools gaining steam but still more behind PostGIS. [PostgreSQL 8.4] Yes - PostGIS is great and lots of spatial functions and fairly efficient indexing and lots of open source and commercial support - ESRI ArcGIS 9.3, MapInfo, Manifold, CadCorp, FME , no geodetic but expect the first version of geodetic in PostGIS 1.5, and fairly robust geodetic in PostGIS 2.0. [Microsoft SQL Server 2008] 
Yes - now built-in, but we aren't allowed to provide benchmarks for obvious reasons. If you use it, you'll really want to install the SQL Server 2008 Spatial Extension tools. The upcoming SQL Server 2008 R2 (currently in CTP is supposed to have Report Builder with map integration features which should be interesting). It has geodetic which PostGIS does not. SQL Server 2008 and PostGIS have pretty identical commercial support for spatial, but PostGIS still has a much larger Open source tool belt following.
Cross Compare of PostgreSQL 8.4, SQL Server 2008, MySQL 5.1 - Postgres OnLine Journal
Microsoft SQL Server 2008