Round trip to Paris. Which way would you upgrade
 

+1
Always try ​​​​​​​and upgrade the red eye for the sleep.

Eastbound out is nighttime flight
West bound return is a 90 minutes longer and daytime flight

For jetlag and best adjustment to destination, I would try to sleep on the outbound. Arriving in Europe in the morning is a killer time adjustment and best to stay awake till the evening (naps on arrival are a bad idea)

So, depends on if the upgrade is for sleeping or if experiencing the cabin and seat is the objective.

Im the opposite. If Im going to be awake, I rather do so where the food is better, the entertainment is better, and I have more space.

Sleeping? Why spend more money to sleep?

I would only do J on the way over IF I was being very disciplined and ate at the Polaris lounge in EWR and slept virtually the entire way over in J. Otherwise, despite the lounge access and the bed for sleeping, I would go up front on the return because it is a longer flight. Sleeping in the back isn't terribly with a 2-3-2 layout, and depending on when in February you are going (since it is off-peak), you could get some extra seats to lay out on.

As an FTer, though, I would be figuring out a way to swing it on both legs. :D

These are tough calls -

This is not an easy sleep flight for most people, it leaves 6:20pm and is only a bit over 7 hours (shorter than HNL-ORD) - by the time you get up in the air, do a dinner and breakfast service, there isn't much time to sleep - even if you can sleep at the early departure. The Polaris Lounge would be the big appeal for me on an upgrade this direction.

On the flip side, if I stayed up late in Paris the night before with an early wake up call - I do think I could sleep on the return, and it is a longer flight.

I'd upgrade both legs

For me it is because I want to hit the ground running with very little jetlag. On the return I stay awake and then can sleep in my own bed, so happy to be in Y but happier in J. CDG is a nine hour time difference for me, but even with six hours (between EWR and CDG) I would still opt for the upgrade on the red eye. We each have our own preferences which is why I think the OP was asking for the viewpoints of others.

I would personally upgrade eastbound (NY to CDG). This is usually an overnight flight, and the jetlag is bad going east, so you will appreciate the extra comfort. You will also arrive fresh in France to fully enjoy your holiday.

The westbound flight is 6 hours, so hardly the end of the world in a Y seat, the lag is not too bad going the other way so more bearable, and also your holiday is over anyway so you might as well feel terrible ;-)

Upgrade the other way if:
-You want to feel better about your holiday ending
-You want to enjoy the Biz lounge in Paris before leaving (If you are flying Delta/Sky team, you have access to AF lounge: free champagne)

Unless UA gets a really fast plane, the westbound is 8 hours 45 minutes

Eastbound. Dinner in the EWR PL and sleep the entire flight.

100% EWR-CDG for the PL... have dinner there (better than what's served on board), a few cocktails if you partake (again, better than what's served on board), get on, put down the mattress pad and snooze the entire flight. Coming home, the 764 Y seat is perfectly nice for a daytime flight.

If you are *G, the *A lounge at CDG is nice, but not on the same level as the Polaris Lounge.

For me, no doubt: Upgrade eastbound. Get up super early that day and have dinner at the airport. As soon as you hear the double ding, go into sleep mode with earplugs and eyeshades. Sleep as much as possible, even through breakfast service if you can.