MacBook with 20" mirrored Cinema Display -- preferred reso..

I'm operating a recent Tiger MacBook connected to a 20" Cinema Display
in mirrored mode as the primary viewing screen.

My objective is not to get a lot more windows or more lines of text or
more data on the screen; it's mostly to get the standard MacBook display
somewhat magnified, with larger fonts or characters on the Cinema screen
than the standard sizes on the laptop screen -- maybe 20% or 30% bigger
-- to compensate for aging and deteriorating eyeballs.

The Display Preferences panel for this setup shows 14 Resolution
choices, from 640 X 480 to 1680 X 1050 -- or seven choices from 1024 X
768 and 1024 X 768 (stretched) to 1680 X 1050 (I can list 'em all if
neeed).

What might be the preferred choices from among the upper half of this
group, assuming I'm most working with text, plus PDFs, web pages, and
occasional graphics or photographs?

I can experiment, of course, but the same visual limitations that lead
to the desire for magnification lead to reduced ability to judge
sharpness, distortion, and the like -- not to mention load on the
computer in changing the resolution.