I'll have to say that I've used both VB - spent $1500 with them upon initial purchase of my car for lot's of rubber bits, fuel pump, motor mounts, (not had any problems with any of it yet), and almost a whole hydraulics system - brake M/C, full clutch hydraulics, etc.... And Moss - through Tony - whom I've spent almost that much for both new Moss parts and many used parts from his collection... I'd still buy from either. Certain electronics stuff are problematic because they come from China - in high quantity, low quality manufacturing environments.
That said. Buy from either, and keep the local guy there, and the major guys there too... It helps keep the hobby less expensive. I've heard it said that we MG guys have it really good for parts availability - second only to VW beetle guys - as the market goes... So I'm in for keeping everyone happy as that goes.