Every FFP has it's strength and weakness. If the particular route you are after works out cheaper for with with USDM, then by all means get it.
On miles alone, yes, USDM charges less, but it costs more to purchase USDM. It costs about 2.2cents each now with bonus (1.88 cents is in the past). So one way to Europe is 60000 with USDM, that's $1320. For LM it's 85000 at ~1.5 cents each (with cyber Monday, we all got less than 1.5 cents, but points + cash still 1.5 cents.), which is $1275. So LM still works out cheaper.
The beauty of LM over USDM is the ability to redeem one way, so gives you more flexibility.
Also note that USDM will be integrated to AAdvantage next year, which you can't route to America through Asia. Not sure about Europe, I know you can't route through Middle East except DXB with Qantas. This really limits the option.
But USDM allows you to mix class, that's a big plus over LM.
At the end of the day, I prefer LM over USDM mainly because I'm a star alliance fan. There just isn't any OW carrier that I really want to fly with. Not even CX. It's personal preference.