I have tried my 901 with 241mm and 170mm dampers.
With 241mm DHX-Air: great for downhill, not good at all for uphill.
With 241mm Marzocchi Roco Air TST-R: great for uphill, great for downhill, including bikepark.
With 222mm DHX-Air: great for uphill - great for soft downhill - the perfect all-mountain-setup for "soft" trails and tour.
I guess it's the same with the 601 as obviously the difference is not so big.
As you want to purchase a 601 along your 901 money might not be the decisive point, is it?
So why not fit your 901 with a decent 241mm coil shock and order your 601 with the 241mm DHX-Air. With the 222mm from the 901 you can build it up as a light-Freerider with around 13kg - and try the 241mm if you want. If you don't need the 241mm anymore just sell it.