Since we do a lot of this here are the differences and the pros and cons of each.

Zooming - Easiest method and should not be done on curved screens. The resolution is ~1920 x 800 or 3840 x 1600 for 4K. Because their is no processing with this image it is the cleanest and most accurate. The downside to zooming is brightness. When you make an image larger it is not as bright. You lose ~25% brightness because your 123" diagonal is actually ~175" diagonal when you zoom for 2.35 if you didn't have the black bars. Also, because you are not using the full 1080 (or 2160) pixels you lose even more. I call zooming the poor man's method of 2.35:1 screens.

Lens - Definitely use for curved screens. While you can not add resolution to the source you can get a perceived resolution increase through upscaling when stretching the picture vertically to fill in the black bars. This makes the picture tall but everything looks correct once you stretch the image horizontally with the lens in front of the projector. The downside to this is it isn't as accurate or pixel perfect but usually only noticeable on test patterns. The plus side is the picture is noticeably brighter.

So while zooming might be more accurate and probably give a sharper image most would appreciate the perceived better detail from upscaling and definitely the increased brightness when using a lens.