I've messed around with this to see if drop rates might be different (like, is it more efficient to do the 10 energy battles or the 8 energy ones?), but I haven't noticed any real discernible difference. If someone has done some serious statistical research on this then I'll take it back, but I think you're just best off going with whatever battle is the cheapest (unless of course one of the more expensive battles has more than one thing you need).
In my experience it varies item by item. There are some items that drop much faster in certain location. Then the second consideration is that some nodes drop 2-3 useful gears, while others only 1. The last consideration is that some hard modes for popular chars also drop what you want, and since they are LONG farms, you'll get that gear anyway...so why rush it?
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Ovechkin {Team Instinct}