Ok well lets see if I can answer these questions. There is just no comparison between having the 2.35 image the same height as the 1.78 image vs the standard letterbox. The visual impact is huge. The difference is like night and day. To me the cost of the lense was well worth it. Could I have lived with the letterboxing? Yes. Could I have lived with the zoom method? Yes. Am I glad I don't have to? Yes! If you can afford it and watch lots of 2.35 material I would say get it. Would I get the lens before I had kick ass speakers, a proper sound system, massive bass etc... No. Would I get a lens before I had and HD disc player? No. But this is just me.
So what is the difference between using the zoom method and a lens to achieve the constant height image? Well for one when zoomed the pixel structure is much more visible and the focus needs to be changed. I really can't explain it but after watching 1.78 HD material when zoomed the image looks like it was zoomed. I know that sounds weird but I really can't explain it much better than that. Using the lens there is no difference between watching 1.78 HD material and 2.35 HD material at full height and width. This is a good thing
That is perhaps the best way to state it. Additionally there is no need to mess with the zoom and focus functions. Press a button on the projector remote, twist two knobs, and presto chango super sized 2.35 image.
For the lens choice it was really easy for me. I knew that the pass through was a feature that I needed to have because I did not want to have to purchase an additional scaler. As to the difference in the models I simply purchased what I could afford.
I hope this answers your questions.