While this is true, SV10 has more plugins, I got several problems when rendering from time to time. At some cases the image is not showing correctly. Ask yourself why you'll have to switch from the 9 V to the 10. At my case I switched thinking the same about the plugins, but after dealing with the interface and the little problems rendering it's just obvious I had to return to 9
I've used both Vegas Pro 9 and Vegas Pro 10 and I can say from experience that I prefer Vegas Pro 10. However, it can be glitchy at times and freezes randomly, not to mention that animating is a bigger hassle than previous versions of Vegas.