The question XNA Framework Vs Dark Game SDK .NET is the same as DirectX9 SDK Vs Dark Game SDK.
One's an engine, the other is an API.
If you want a simple framework engine for Managed Languages that utilises DirectX; then look at the DirectX framework Utility Toolkit. It's not quite an engine, but it does take away much of the common background coding - when combined with Managed DX Extensions, does provide a much easier I/O system.
Personally speaking, I'm not really that impressed with the Dark Game SDK. It's a library engine designed to be used in COFF capable languages in particular Visual C++, yet it is identical in use to DarkBASIC Professional.
The same is extended to the .NET version, given it's designed to be used purely in OO Languages; the fact it has a very limited namespace useage, and doesn't utilise classes for the languages capable of running it.. honestly makes it a little pointless.
Personally I would use a 3rd party engine like TorqueX or DXLib (http://www.gpwiki.org for more options) as they will provide you with a much better designed engine that will integrate with your code better.
Intel Core 2 Duo E6400, 512MB DDR2 667MHz, ATi Radeon X1900 XT 256MB PCI-E, Windows Vista Business / XP Professional SP2