Averill will hit for a higher OPS at home (1.621 vs 1.556 on the batter card), Snider will run better (1-16 plus a SB threat vs 1-11 with no SB) and have slightly fewer non-throwing errors.
...And Snider has a better clutch and less gbA. Plus, Averill's arm will be tested more frequently than Snider so he is likely to have more throwing errors in addition to more non-throwing errors. The peripherals make Snider a better card in my opinion. This said, with Stearns who can handle centerfield, the better choice among lefty bat who can play outfield in the 10M-11M range would be Musial. I would try to do a straight up trade to get him. Musial is a natural lf, so you don't lose money playing in left field someone who was priced to be a centerfielder.
It doesn't seem to me that better arms necessarily throw out more runners, from years of perusing their assist and of dp's. The major value in good arms seems to be in deterring opponents from even trying for the extra base.
It is true that runners are less likely to test a -2 arm, and this has the consequence that more runners may be thrown out by a 0 arm. But the overall percentage of runners who are safe when tested is still lower with the -2 arm, so overall, there is still a slight advantage to have a -2 arm, in my opinion, but I agree it's rather small. What really hurts is when you have a +2 or worse arm in centerfield, the relationship between arm value and defensive impact is then exponential.
The clear advantage of having a -2 arm vs 0 is when you have a fast runner at second base and other bases empty, with a slow hitter hitting a single, when it's still early in the game. In such scenarios, the play is almost automatically at the plate with the hitter sticking to first base. Having a -2 arm will increase your out chances by 10% in most cases. If the hitter is very fast, then it's more likely that the throw will be intercepted, regardless of the arm value.