Go to SeatGuru.com and check out the difference for E+ vs. E- on the different metal. As goplaces has just noted, E+ nets you more room.

A quick glance over there shows the following: (added AA for your comparison)
AA 757 in Y = 31" pitch, 17.2" width
UA 757 in E+ = 36" pitch, 17" width
UA 757 in E- = 31" pitch, 17" width

AA 777 in Y = 31-32", 18.2" width
UA 777 in E+ = 35-36", 18" width
UA 777 in E- = 31", 18" width

Also worth noting that if you are comped to at least Premier Executive (1P) then you will have the ability to book exit row seats at ticketing -- generally resulting in even more room -- sometimes significantly more.