The value of MR points and UR points depend on what partner you transfer to, what you redeem for, and whether you can find "saver' availability or not. This is all hard to determine ahead of time, since you don't know ahead of time who will have avaiability, and then where you can redeem for that availability, and how much it will cost (both in miles and cash for fees + taxes).
If you don't know how to use international transfer partners, and only look at Delta for Amex MR and United for Chase UR, then it depends on the season. In "high" season, Delta is likely to be useless. In "shoulder" or "low" season, Delta has a chance perhaps.
But it depends a lot on specifics, not just dates but also destinations. For example, there are some places in Europe that only Star Alliance can get you all the way (northern Norway, for example), and in that case you have to have miles in the right alliance. Other places, like London or Paris, anyone can get you there.