Watching trends without an analytic platform, I have seen that over any period of time longer than ~1 day, ETH seems to perform better. It is common to see periods of an hour, up to a day, where BTC does perform better. (For this reason, not more than just a few weeks ago, I took most of my tiny collection of BTC blue chips and traded them in for ETH.)

It would be hard to believe that nobody has tried writing arbitrage bots. I would probably be overreaching to say that this effect is due to a large number of arbitrage bots (and likely also non-robots), but it seems like there are clear opportunities for arbitrage and likely that there are skilled individuals taking advantage of these chances on the regular.