You are well positioned in 21C. Row 21A/C/D/F are the best Y seats on the UA 752. Lots of leg room, and they have recline unlike row 20 which are fixed. There is enough room to get by your seat mates without them having to get up. I would choose 21A as I could lean towards the window side but that is personal preference.
I once did a EWR-LHR r/t in 21A and had 21B/C vacant on both legs. Quite decent circumstances and a rare UA nicety.
If you were stuck in an E- middle seat I would suggest you more seriously consider the upgrade. But if you can tough out a relatively short overnight flight IMO better off staying where you are.
SL