Let's start somewhere. Let's look at glide performance.  You want a glider that will have the best glide ratio and the "best" glide speed. The best glide speed will vary. The best glide speed between thermals may be slower than the final glide speeds (it depends on tactics). You'd like a glider that would have a better glide ratio at all "higher" speeds.

Let's start here: http://ozreport.com/forum/viewtopic.php?t=10475&start=3 Brett Hazlett writes:

However, from a pure performance perspective, after lots of tuning, test flying and geek sessions on my computer, I have come to a simple conclusion.

Span = Performance

Take a look at his whole article. And his follow up articles.

Let's look at the spans:

Each span for each model is given in feet and then meters. If we were to look at just span as the determinant of performance (and again read Brett's article, which makes the very necessary qualification that this does not necessarily apply across different designs) then we would rank the gliders as follows:

10.7 - Aeros Combat L 15, Aeros Combat L 14
10.5 - Laminar Z9 14.1, 14.8
10.4 - Moyes RS4, S5, S4.5, Airborne C4-14, Aeros Combat L 13
10.3 - Moyes RS 3.5
10.2 - Wills Wing T2 - 154

Have I done this correctly?

I'll get more into the issue of span, aspect ratio and wing loading going forward. Of course, the pilot can't make his/her choice just on the basis of span, otherwise hang gliders would have spans of 15, 18, or 20 meters (although there are obvious practical considerations here).