A friend commented in a previous post : "Most of the time you trade EJ sometimes you trade EU. Do you watch both pair at the same time? When you trade EU how is that experience different from trading EJ? Isn't it that EU is very slow, unpredictable and spikes too much?"
I always have open charts of both pairs and examine the setups on both. I usually choose the pair with the bigger volatility. I have an ADX indicator (1 min) on a Market Analyzer in Ninja Trader which calculates the volatility of the two pairs for me. I choose the pair with the bigger ADX reading, if I can "feel" the corresponding setup. I also avoid trading when the pair has ADX<20. I "discovered" that, most of the times, when one of these two pairs ranges, the other one trends , and vice versa. This is due to the triangular correlation and fixing between euro, usd and yen.
When euro/yen stutters, euro/usd is FASTER, in most situations. No pair is unpredictable in the (very) short term, no one is predictable in the long term. Hope this helps.