High compression vs. low compression

["DiFede, John" <Jdifede@MediaOne.com>]

I'm planning an engine rebuild (18V- 801AE?-- 1975 production year block)
and was thinking of changing the engine over to the factory high compression
spec of  9:00:1.  Is the piston shape (dished vs. flat)  the only difference
between low and high compression?   Also, what benefits would I see in a
high compression engine?.  What about any downside?  

John
73 MGB 
72 MGB