pascal wrote:
I am leaving at 420m above sea level (1300ft) and have the possibility
to drive within 20mns to a 1100m (3300ft) location

q1: how is the highest point likely the improve observation through an
SCT scope of the 8-10" range

All other things being equal (like that ever happens) you're under about
7% less at atmosphere. You *might* notice a difference. You're better
off scoutting out the darkest spot within 20 minutes rather than
emphasizing the highest. Could be the same spot, but dark is what's
important.

q2: how does it affect the choice between a SCT 8" and a 10" ? when
price and weight are not criteria for the choice. I mean for example at
the highest point both will be very good, therfore no need to go for
the 10", etc...

There Is No Substitute for Aperture
I live at 7200 feet and want to trade my 11" in for a 12.5" (Also SCT to
Dob Newt, so there's more to it than just inches). The Keck telescopes
are at 14,000 feet. Big and high.
Altitude doesn't mean dark, or good seeing. Visit Denver :-PPP

Shawn