Be aware that if you use the Mitsubishi method, when you watch 16:9, the projector has to scale the image down to a ~1440x1080 image so the lens can stretch it back to the correct size. You are literally eliminating 25% of the image information just for convenience.
For any anamorphic lens option, the projector has to digitally stretch 2.35 material 33% vertically. The quality of the result will depend on the scaling ability of the projector. People keep saying you get 1:1, but that is not correct. What they mean is that you are utilizing the full image panel in the projector, but is that really a good thing? The portion of a 16:9 image used by a 2.35 format is 815 pixels high, so stretching it out to 1080 is not 1:1, it's 1.33:1. In my opinion any time you do digital processing on an image when it is not necessary only adds risk of making it worse. That on top of the fact that you introduce pincushion distortion and chormatic abberations makes the lens option a two thumbs down in my opinion.
The biggest detriment to the zooming method is that you are using 75% of the image panel, and therefore 75% available light, on a screen that has 33% more area than a 16:9. With a lens, you use 100% of the available light on a 33% larger screen. The PT-AE3000, however, is a rather bright projector and can still provide enough light output at that screen size. This is especially true in a light controlled room. In fact, you might even want to turn down the lamp brightness in 16:9 mode to provide the same on screen light density as you get in 2.35 mode. You would even extend the lamp life a little if you did.
Lastly, since you are zooming, each pixel does infact occupy more screen area in 2.35 zoomed than 16:9 regular. However, if you do the math for a 50" high screen, a 2.35 zoomed pixel is only .06 inches wide and a 16:9 pixel .046 inches wide. When you are sitting 10 feet from a screen, i challenge anyone to tell me the difference between the two. Do a little research into human eye acuity and you'll find that you need to actually sit 6 feet from the screen before a person with 20:20 vision could discern two adjacent pixels at that size.
Conclusion....buy the Panasonic, set it up for zooming and enjoy. If you honestly think you can gain by adding a lens...you can still do that later down the road.