I have a Jambo and have had a Lang and I don't think you can go wrong either way. The real difference in the two is the insulated firebox on the Jambo and its 2 door design rather than one. The single door may be an advantage if you plan on cooking a whole hog or very large cuts. Other than that, compare shipping costs and then make your decision.
There are some small differences in how you cook on each pit simply because one is a standard offset (Jambo) and the other is a reverse flow (Lang). But, that's no big deal. The Jambo will get better fuel "mileage" because of the insulated firebox but for home use and the occasional competition that will probably not be a significant difference.
Operation BBQ Relief Founding Member - I am Obsessive Compulsive about BBQ. Google it.