While ground dimensions can definitely make a difference to game styles/plans I don't think weather conditions are as important.
In the bad old days of the 70s, 80s and probably very early 90s when some grounds were bog heaps it would certainly bring goods teams back to the lesser teams (Saints watering Morrabbin comes to mind when they were woeful) but these days with the excellent drainage, if a team is mature, strong bodied, skilled and well drilled (all things that St Kilda are)then I think it makes them even better than the lesser teams.
The only real factor IMO is wind affecting field kicking and goal kicking but it is the same for both teams. Geelong is experienced in playing in windy conditions but it doesn't really matter as being windy at the MCG is a lot different to it being windy at Kardinia Park. At Kardinia Park the wind tends to blow one way or the other in a pretty straight direction whereas at the MCG it tends to swirl around.
Bring on the finals, I love September, nice sunny days and the smell of cut grass.