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.