Upon further reading of the charts, ER2 does seem like a better deal for day trading with a breakout strategy. The intraday swings are typically more than 10 points (sometimes 30 or more) and ATR is about 1.5 points only. Furthermore, there seem to be fewer fake breakouts in ER2. See figure for example.
 Also, 1200 contracts / bar seem to be a good starting point to test some strategies. I'll work on this more this week.