Hi--

>1) So, based on your testing of both 75-300 and 70-300 samples, would you
>recommend without hesitation the old zoom design over the new one based on
>optical performance?

Yes, based on my experience, No, based on other's comments...
(Maybe I saw a particularly good 75-300... [but maybe not, given your observations...;-].)

>2) You rated the old 75-300 a 4.5, what number would you give to the new 70-300?

3.3-3.8 for the new, 4.2 for the old...

>3) How well does the new zoom work with the 6T diopter (the old design works
>very well for me)?

Didn't try it... But if the new one works well...

>4) Does the new zoom's filter thread rotate during focusing (the old one does)?

I don't remember. (I don't use polarizers...)

>5) Does the new one zoom-creep when pointed directly down (the old one does)?

No, but I never wear a camera on a neck strap, so that doesn't matter to me...

>6) I know the new design have more plastic construction and much lighter,
>but do you think it is assembled and might hold up not as well as the old
>design?

Probably - it seemed well-made, if light.

>7) A side question: I've made a comparison test between the 75-300/4.5~5.6
>sample and my 70-210/4 Series E by shooting a color map at different focal
>lengths, apertures, distances, and with a 6T attached. I've found their
>resulting slides show similar sharpness, contrast and saturation when closed
>down somewhat. However, the 70-210 Series E showed significant light
>fall-off accompanied by a loss of edge sharpness (especially at 210mm) when
>tested wide opened. Whereas, the 75-300 showed very acceptable fall-off and
>edge sharpness levels when shot wide opened. So far I'm quite happy with the
>75-300's performance and feel confident to shoot it wide opened (which is
>important given its slow speed). Any comments on my observations?

Yes - if the 75-300 looks this good, keep it! ;-)
BTW, my experience with the 70-210 is that it is better than you indicate
(sharp to the corners wide open throughout.

>I've come to you to confirm my inclination for the old 75-300 over the new
>70-300. I still have a few days to return the lens if I decide otherwise, so
>I appreciate greatly if you would help me with any of my questions. Thanks
>in advance!
>
>Frank Lee

I think you have already decided...;-)