Are the hotels of similar quality and do they have similar customer satisfaction levels? I would check that first.

After that, this is a difficult question for me, too. The number of miles mean practically nothing. One thing you can do is look at converting the points to a similar airline. For example, 10k Marriott points will get you 2k AA miles, but 10k Hilton Points will only get 1.5k AA miles. So that suggests that Hilton points are roughly worth 75% of Marriott points, but that is a very rough estimate. The Marriott is a slightly better value than the Hilton using those estimates.

But converting to airline miles is rarely a good use of hotel points. A better question is, what else can I do with the miles? Based on where you travel, the types of hotels you like, and your remaining balances/ability to earn more points at those chains, which miles are you more likely to use in the future?

For example, if you rarely stay at Hilton hotels and you have exactly 40k Hilton points, use those. But if you have 50k Hilton points, and use 40k, it will be pretty hard to use the 10k remaining points for hotel nights, unless you like Hampton Inns in rural or industrial areas. If you have 125k Hilton points, those are much more valuable used on a 4-night VIP or AXON award, so to break them up into 40k and 85k chunks really decreases their value.

I don't know enough about Marriott to give you any pointers there, so maybe someone else can jump in here...