At 25k miles on UA, you do get complimentary E+ seat selection. And with the announcement that CO will also have E+ seating in the future, that gives many more options for E+ seating on various routes (providing the schedules are not reduced significantly).

At 25k-50k miles, complimentary upgrades on UA are difficult - especially on hub-to-hub flights. However, if you keep building your miles on UA, then you have a good option for using miles to upgrade on international flights.

If you will only fly less than 50k miles per year, you might consider using UA to get to Premier status to keep E+ and use VX for other flights where you can pay a fee to upgrade to MCS or F (providing seats are available).