I've only see pictures and specs on the ES-137; haven't played or even handled one. Without playing it, I lean toward the ES-339 because of the thinner body. I love the semi-hollow body guitars for their tone and own an Ibanez AS86 (which is basically a Gibson ES-335 wannabe). It's a great guitar, but if I could make one improvement, it would be the body size...hence my attraction to the ES-339.
I also have an Epi Wildkat which is smaller than the ES-335 and has P-90 pup's. It's a great Rockabilly sounding guitar but also very versatile. Again, the hollow body gives it great tone.