It depends on which airline you are looking to use or if you are intending on paying with points. If paying with points, it depends on how many points you have to play with and which cards you have. If the flight is say $1000 RT and you are paying with UR points via a CSR account then you would need 66,667 UR points to pay for the flight. If you don't have a CSR and all you have is CSP or CIP then you need 80,000 UR points. With MR you would need 100,000 points. If you have an Amex Platinum business card, you'd still need 100k MR but in a few weeks after paying for the flight with your 100K MR points you'd get 35K put back into your account.
If you are transferring 1:1 to an airline -
UR transfers to United (instant), Air France (instant), British Airways (instant), Singapore (12 - 24 hours),
Virgin Atlantic (instant), Korean (instant), Iberia (instant), Aer Lingus (instant)
MR transfers to Delta (instant), Air Canada (instant), Air France (instant), Alitalia (instant), ANA (48 hours), Singapore (12-24 hours), British Airways (instant), Cathay Pacific (48 hours), Emirates (instant), Etihad (3 business days), Iberia (~48 hours), Virgin Atlantic (instant), Hawaiian (instant)
Some of the airlines listed above would allow you to book flights to Europe directly on the airline. Some, you'd have to use the miles to book on a partner airline.
Before choosing which currency to use MR or UR it is important to discover which airline you will likely be flying and how many miles you will need to fly that airline directly or via a partner award. Once you know that, then you want to get an idea of what a flight might cost and if it will be less points to get the flight by an award ticket or by paying with points. If you decide paying with points is more desirable, then you need to decide if you would be able to amass more points faster/easier with Chase or with Amex and go from there.
We value both fairly equally. But I am far more conservative about spending MR points than I am about spending UR points. Currently we have a little over 408,000 MR points and a little over 565,000 UR points. Our MR points come mostly from credit card sign up bonuses. Our UR points come from spending on bonus categories. We are

/24 and the last time either of us got a Chase UR card was the CSR before the loophole we used to get it closed. This was September of 2016. We are both resigned to the fact that we will never be able to churn any Chase card. Ever. We love Chase and are quite happy with the cards we have and we have no intention of every rocking that boat for any reason.
Personally, I find it easier to accumulate UR points but that is because we have 3 CF cards between us and max them out each quarter. We each have our Ink+ and use it to purchase Disney gift cards for 5x at office supply stores. Most of our UR earning comes from purchasing Disney gift cards. We just booked the Japan ABD that was released last week with the post trip Disney park package. It is easy enough to look up the info on ABDs website so it is no big secret what the trip costs. Opening day price for this trip for the two of us was a little over $25,000. We intend to pay for it with Disney gift cards at 5x so that trip alone is going to earn us 125K UR points for next year. Add DVC dues on 1300 points, annual passes, a
Disney cruise here and there and you get the idea. I won't think twice using our UR points because I know they will be replenished soon enough. MR points on the other hand, I always pause before using them.
I think it is important to take into account not only which points you might use and how you might use them but also, how easy or how hard is it to accumulate those points. This will depend on your ?/24 status and your spending patterns. I hope this helps