I don't currently have a scale worth a hoot to weigh the "standard" Blackhawk witk alloy grip and ejector rod housing against the all steel Flattop, both in 45 Colt with the same barrel length.
Subjectively they feel about the same weight but the Flattop seems better ballanced to me while the "standard" Blackhawk feels more muzzle heavy. I do have an all steel 5.5" SS Bisley that is MUCH heavier than the 5.5" Flattop.
You might want to check the listed weights on the Ruger website.
The Flattops feel much closer to Colt SAA and seem handier to carry. There is no doubt the "standard" Blackhawk 45s will handle heavier loads than the Flattop 45s. I just like the feel of the Flattops...