The 2020 is 13.5 gpi, and the 2117 is 12.0. The 2020 has a deflection of .426 and a spine rating of 77#. The 2117 has a deflection of .400 and a spine rating of 81#. That's Easton's data and the spine weights are measured on 26" centers.
The first two numbers in an Easton arrow size denote outside diameter of the shaft. 20xx means it's a 20/64" (5/16") shaft. 21xx equals 21/64" diameter.
The second two numbers denote wall thickness. The 2020 has a thickness of 20/1000" and the 2117 is 17/1000" thick.
The 2020 will make you a little heavier shaft, but the 2117 is a tad stiffer, owing to the larger outside diameter.