In reply to George1924:
I've had several trips to the Saastal, all in September and I've yet to lose a day to bad weather. Still plenty of snow but clean rock as well, on the routes I've done, at least.
In terms of the climbing alone I'd say the Saastal would suit you better. Bear in mind though that, at least in a normal year, it will be a lot quieter than Cham. No queues on the hill but no one in the bar, either.
Routes: Alalinhorn (F) is a snow plod but short from the Metro; more interesting from the Britannia hut (great hut, btw).
Weissmies (PD) is a cracking day out done as a traverse from the Almageller hut. Easy rock scramble to the top then glacier down the other side to the cableway. Stop at the halfway station and hire Trotties to go back to Saas Grund. Silly but good fun.
Longer, more serious routes on the other side of the valley.
For acclimatization you've got two good VFs, a nice walk up to the Italian border with a superb view of Monte Rosa if you get up early and several good hut walks.
Cherry on the cake is free lifts in the valley if your accomodation is in the valley (Metro Alpin excepted, I think).