100k UR are more valuable than 100k Marriott. First of all, at the very least you can transfer the 100k UR to Marriott and have 100k Marriott points. But you can also transfer them to many other travel partners, including Southwest, United, Hyatt, etc. Also, if you have the Chase Sapphire Reserve, you can get a minimum of 1.5 cents per point when you spend UR through Chase travel portal. For general redemption value, I would say that UR are worth at least twice as much as Marriott points.

To pay for a flight with Marriott points, I believe you would have to transfer your points from Marriott to SPG (at 3:1) and then to your airline of choice. Even if you count the minimum spend and the authorized user bonus for Marriott, you would transfer 112,500 Marriott points to 37,500 SPG points. With the SPG airline transfer bonus (5k for each 20k transferred to an airline), you would end up with 42,500 airline miles. Assuming you put more spend on your Marriott card, you might get that up to 50,000 airline miles.

If you are willing to fly with one of the CIP's transfer partners, you could transfer 50k UR to 50k airline miles and have >50k UR left over (counting min spend).

I hope I've answered your question. My question for you: Why not get both?