If you ride like I do, the LX is the way to go if there's any question. I'm hard on a few parts: pedals, 44t chainrings, rear derailleurs, and derailleur hangers. I tend to be a tad frugal on the parts I screw up and I rarely have the budget to replace something high end on short notice.
My understanding is that the LX still ships with the aluminum middle ring. If not it has a plain steel ring which is better but heavier. The middle ring is the most used so it should be made out of something tough. Aluminum isn't tough. It wears quite rapidly.
The XT middle ring is a carbon/steel composite. The teeth and bolts circles are steel, everything else is carbon. You can't buy these aftermarket (which is why I have three chains ready for replacements). The ultimate is the XTR crank which has a titanium/carbon composite middle ring.
I'm thinking about getting a titanium granny made. After market titanium guys don't to ramps and pins. But, grannies don't need ramps. It's a perfect application for an imperfect solution. Though, I'm look forward to seeing other companies come up with molded organic (carbon or plastic) ramps on titanium chainrings.