I think collectors and appraisers like the first and simpler model, and because of this the 335 is considered more "collectable". The fancier models 345, 347, or 355 (BBKing) are more complex and less desirable, a conclusion that doesn't really make sense to me. Even with the lack of binding and the Ebony fingerboards the 335 is still king.
Here is what is commonly believed about the ES-335, as quoted from "Antique Vintage guitar info" website:
Description: Gibson ES335 Electric Thinline Archtop guitar.
Available: 1958 to 1981 (but reissued by Gibson as a reissue dotneck in 1981)
Case: Brown hardshell case with a pink lining was the top-end Gibson case from 1958 to 1961. Then in 1962 the case changed to a black outside with a yellow plush interior. Also available with a low-end aligator cardboard case.
Collectibility Rating: 1958-1960 "dot" models with "large" neck: A+, 1960-1962 "dot" models with a "thin" neck: A, 1962-1964 "block" marker models: B+, 1965 to 1969 "trapeze tailpiece" models: C+.
Production: 1958:317, 1959:592, 1960:514, 1961:886, 1962:876, 1963:1156, 1964:1241, 1965:1750, 1966:2524, 1967:5718, 1968:3760, 1969:2197
General Comments: The Gibson ES-335 guitar with its semi-hollowbody construction is a great guitar. The solid maple block down the center of the body minimizes feedback, but the hollow body wings gives good sustain, tone and weight. This model is most desirable with a stop tailpiece and a large neck size. Also the dot fingerboard inlays (aka "Dot Neck") version is also desirable. Additionally the long pickguard (pre-1961) models are also nice. Bottom line, the 1958 and 1959 models with stop tailpieces are considered the best. Alternatively I like the 1963 and 1964 models with a stop tailpiece, because the neck shape is nice. A Bigsby vibrato on the 1958 to 1964 models hurts the demand of this guitar. Also the 1960 to 1962 style "thin" neck also hurts demand (compared to the earlier "large neck" models, but this is a general fact of all Gibsons of this era).
In that case, the 347 is the most simple model with an ebony board. It doesn't have a mahogany neck as the others do, but a 3-piece maple neck, same as the BBKing model. The absence of the Varitone or a Bigsby vibrato is a mark for simplicity. The TP-6 tailpiece puts it up to the level of some pricier tailpieces I've seen on L5 models. Hang on to these guitars and I think they will become collectable.