All Hollowgrams have the same interface for the spider/spiderrings, so you don't really have to worry about which version you get.
The original Si was silver. It is still super light and stiffer than DA7800. I ran a few of these, and still have one on my cross bike. You can find these used pretty cheap, and it's a great value.
The SISL was the second incarnation (~2008). It was originally silver, then made in black. It was renamed as or replaced by the SL at some point. They may not be exactly the same crank arms, but they are basically they same thing. The weight and stiffness should be pretty similar.
The SISL2 is the latest version. It is the lightest and most expensive of all of them.
There is a new crank that came out in the last year or so that is referred to as the SI. I am not as familiar with it. I am assuming it uses the same interface as the others.
The biggest area of concern at this point is the Spindle. Though it is the same for all versions of Hollowgrams, the original spindle was shorter - 104mm - and only worked in BB30 frames. The newer spindles are 109mm and work in BB30A frames. I'm not exactly sure when the spindles changed.
If you don't mind the Silver and money is a concern, the original arms are a tremendous value. If you want black, the SISL/SL is going to be much cheaper than SISL2. If weight is your concern and money not an issue, go SISL2.