TweakTown uses a different method for testing CPU heatsinks which allows for an even playing field across all product tests. We feel that by using the same ambient temperature and strict lab-like testing procedures we are able to accurately compare one product to another. More information on our testing procedure can be found in the T.E.C.C. article here.
At idle I found that the XiR-2300 performed just a little better than the XiR-3500, but the results were reversed at load. Either way, both coolers are very good, and are amongst best we have tested so far.
The XiR-2300 and 3500 use the same fan and have the same specifications. I found that the two varied a little, but the amount was so small it can be written off. The numbers pretty much equalled out to being very quiet at just a couple of feet away.
On a side note, Nexus does own the title of quietest fan in our testing from when their Basic Series Fan was used in combination with the Asus Triton.