They really are two VERY different plug-ins (As noted in the very good contrast of the two offered above)
In my experience, having both available - I almost never use the Alpha Comp in a MIX context not even the aforementioned "Mix" version. However, I do frequently use the Alpha Master Comp in mastering situations frequently.
Conversely, I have been using the VSC-2 all over the place in a MIX context with wonderful results.
Also, it's worth noting the Mpressor from Elysia which finds it's way into my sessions quite a bit on individual instruments (and the Drum Buss) and is a very interesting compressor. (I do own a hardware version of the Mpressor and while not a spot on emulation it is indeed quite good).
If you are not Mastering often - Or should you already be covered in your mastering needs - I would recommend the VSC-2 were I to choose one of the two.
Try them both if you can - it's pretty hard to go wrong with either one.