Difference between VGA and TV output

[Dave Chapman <dave@dchapman.com>]

I am about to buy a system to use as a Linux-based STB, and have a question.  
What is the difference in picture quality between using the V output of the 
DVB card and using a TV output from a PC's normal video card?

I'm assuming that the DVB output will be of the higher quality, but can 
anyone comment on their experiences?  

I'm asking this because the system I am planning on buying (an Allwell 
STB3036N from www.gctglobal.com) has the ability to overlay alpha-blended 
windows on top of the VGA screen, which would seem to solve the OSD 
limitations that I've been reading about on this list.

Best wishes,

Dave Chapman.