I own a SansAmp PSA-1 and have used a Vox ToneLab side by side. Vox does certain models really really well; for example Vox does a better Marshall than any current Marshall. But on the SansAmp, every 'model' (or preset that emulates a model), if you've heard the real thing, you can tweak a couple knobs and get it to sound just like the real amp.
Vox has one thing in common with Line6 that SansAmp does not: SansAmp sounds like the actual amp as when you are standing in front of it. Vox (and Line6) sound like a very nice recording of the actual amp.