importance of altitude

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

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...

Thanks.