Given the amount of points/miles you are talking about the question really isn't which amount is worth more. The real question should be which will you be able to maximize the most.

How will you use your Delta miles and your Marriott points? The extra 6K miles could possibly give you another domestic roundtrip ticket. I do believe there are some Delta awards that are 35K each. If you were planning on using one of those, then the extra points might not be needed. The extra 15K points could give you an extra night at a Marriott hotel. Is that more/less of a benefit than the extra flight ticket?

You can also look at future benefits. Are you trying to save points for a Marriott Travel Package? If so, then the 15K would help toward that goal. Not to mention that a TP award gives you airline miles at a 1-to-1 basis. Thus your 15K Marriott points would be 15K airline miles. But that is only an issue if you are trying to save for a TP award.

One more thing to muddy the waters is what is your future earnings potential for each of the programs? It maybe that you can more easily earn Marriott points, thus maybe the Delta miles are better for you. (Of course if you can earn enough Marriott points for a TP award, then your back to Marriott points again.)

I don't always look at which one I can squeeze the most monetary "value" out. I tend to set award goals and then try to maximize my earnings to achieve those goals.