Miles in a United MileagePlus account are worth about 2 cpm (cents per mile), nominally. Reasonable valuations range from about 1 cpm (if you're not willing to invest a lot of time in maximizing the value you get from them) to 3 cpm (I consistently redeem at about this value, but it takes effort).

So you're looking at between $30 and $90 in value there. I don't know how much a Miles-and-More mile is worth, but I'm guessing it's not much less. So if you have the option to put the miles in your LH account for free, that's probably the best deal. On the other hand, if it's gonna be $25 either way, then you could go either way. And just letting the miles slip out of your hands is probably not cost effective, since they're worth at least $30.