They sound and behave differently. While the VSC-2 has a very distinctive sound/color, the Alpha compressor is more versatile and can go from "clean" compression to "colored" results. The Alpha compressor has a "Master" and a "Mix version which makes it very versatile as I said. The Niveau Filter and the "auto-fast"" features makes it usable in many situations (the VSC-2 also have an auto release but Alpha's "auto-fast" is more predictable and "grabs" the sound in a pleasant way). If I had to choose one I'd definitely pick the Alpha compressor. The VSC-2 doesn't work in some cases (but it's awesome as a buss compressor in my opinion) while the Alpha compressor is hard to hear it sounding bad (unless you do something wrong). I should also mention that the VSC-2 has one particular feature which makes it very interesting: "soft ratio". Some sort of smart "ratio" that is increased automatically with the input level which helps to get smoother results on some sources (at high gain reduction levels).
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.