Heres my take on the Omega, having driven both. Fold the back seats down flat on the Estate, loads of carrying space, with seats back and getting stuff under the load cover, probablky the same room as saloon.
Driving, - bearing in mind your facing foward, no difference, Estates do give the impression of bigger interior, more roomy.
Parking, - probably easier to park an Estate when reversing.
visibility - Estate better, see Parking above.
noise, - Estate slightly nosier on a Motorway (if empty), not by much though.
fuel tank size, - No difference.
mpg, - Personally Ive always found the Estates slightly better on a run.?
different insurance group, - shouldn't make FA difference at our age.
less weight on the back end unladen, - if you really push them to their limit, the back end can be a bit twitchy.
any longer (for garage space), - very slightly, you wont notice.
rear legroom - probably the same, better head room for tall people and better width room on Estate, 3 people in the back have more room over saloon.
different rear shockers/springs to allow more load carrying than a saloon, - Estates perform better fully loaded over fully loaded saloon.