For better or worse, the "L" series are built to similar design/level as most of the MTD or AYP machines out there - (possibly excepting the CCs) and nothing necessarily wrong with them, but the LT series has many small improvements that *should* result in greater longevity and better 'repairability'.
Browse the JD Parts pages where there are exploded diagrams of the whole machine, and you'll see things like bushings on the LT where the L has nothing, and ball bearings on the LT where the L has bushings. Welded frames versus bolt-togethers. You'll find as you go up the Deere line into the LX (and on into the GT, GX, X... etc.,.) series the 'sophistication' grows and things are built using simply heavier duty components.
Only you can make the call as to whether those 'upgrades' are worth the difference in cost for you, but the LT160 seems to be getting awfully popular as a decent compromise between price and enough small improvements over the typical lawn machine to be worth while. The LT180 has a nicer engine and 1 more hp, but it comes at quite another jump (over $500 extra) in price. I don't think there's any other difference between the two besides the engine.
If you do go with an "L", consider getting it from a dealer rather than the box store. It should be the same price anyway, and at least you'll begin a dialogue with the shop that's going to have to service it if you need any help.