Is there likely any meaningful difference between these parts that are just $0.20 different in price?
ACDelco 195 degree high flow thermostat
https://www.rockauto.com/en/moreinfo...=6123&jsn=6123

Gates 195 degree high flow thermostat
https://www.rockauto.com/en/moreinfo...80314&jsn=6135

Both are listed as premium and supposed to fit my 1996 XJ 242.

I've been sold on Gates hoses since before I was aware that is what's been on my radiator since replaced in about 2003 and some recent small stuff is from them too. ACDelco fuel cap is looks and feels very high quality, but that was kinda pricey for a fuel cap too. Should have been made out of plutonium.

The picture looks like the ACDelco is a high flow the gates doesn't look like a high flow to me and reading the specs it doesn't say it a high flow . High flow thermostats have a larger opening so more coolant can go though the opening

That was my bad prose there putting "high flow" on the Gates.

Can't tell if either has the little hole in the flange that helps with purging(?).

Anyway, high flow is the preferred version it seems in this forum. Any downside to that?

Edit: Also noticing some say "fail safe". Aren't they all supposed to fail open, or is that some myth?

They are definitely supposed to stay open when a failure occurs. I would go with the gates. Unless the rest of your system is set up for high flow its not going to make a difference. If your cooling system is working correctly there really is no need for the high flow bs

Just as well, the ACDelco one is not available and it was automagically removed from my cart.

And a bit of, apparently, parts professional humor:
Features including jiggle pins and check valves for air venting, seals, and gaskets make these thermostats equivalent to the original equipment parts.

Down in the specs:
Jiggle Pin/Check Valve | No
Number of Jiggle Pins/Check Valves | 0

No need for the jiggle pin if you are worried you can always drill a 1/8" hole in the tstat, make sure it is installed with the hole at 12 o'clock