I used to only watch ES, then NQ, then back to ES.
Now since i only watch 2 levels there were a lot of days with little to no trades, hence why i switched sometimes.
Now i have added a lot more instruments for which i also watch the same 2 levels.
ES, NQ, YM, RTY, ZB, ZN, ZC, ZW, FVS, FESX, FDXM, FGBM, FGBL, FXXP, FBTP and finally FOAT and every now and then i also do a spread trade, mainly on ES/NQ.
Might seem like a lot, but i only watch 2 levels for each instrument so there is plenty of time just staring at the screens with nothing to do. Today i had 4 trades over the course of around 8 hours, so lots of free time.
I wouldn't suggest watching that many instruments if you are still figuring out how to trade or what to do, but once i found my strategy i started over trading while watching ES or NQ only, trying to get in on every single movement around my predefined levels instead of waiting it out and getting in on the confirmation, in fear of missing the only signal i had during the day.
I knew the system was solid, but this was killing my performance. Now that i have added all those instruments there is no more fear since it is very likely there will be another trade later on in the day.
So my conclusion: watching only a few instruments is probably best if you are trading very active and are in and out a lot. Watching more instruments is probably best if you are trading only a few times a day to get as much signals as possible and have a lot of time to analyze all charts, both to avoid over trading in fear of missing out as to avoid maybe even multiple days without any decent signal.
To answer your actual question

.. I would pick NQ if i was looking for a lot of volatility and a lot of movement to be in and out in low timeframes. I would pick ES if i was looking for more "mellow" price action. ES tends to respect its support/resistance levels, lows of days etc much better compared to NQ, NQ often does retest and retest lows/highs and keeps stopping you out, before finally moving in the direction you were actually trading in, ES doesn't do that. It just hangs out around a price level in a few ticks range instead of bouncing all over the place.