The biggest difference is the dynamic drum, when it's working correctly. The HR-S9600, 9800 and 9900 are better VCRs.
But is there a substantial difference? Well, it's not like going from plain VHS to S-VHS with TBC. You may see some mild improvement in tracking and stability. It really depends on the tapes.
The RAM differences are essentially meaningless. For one, the RAM types are not all the same. For the same generation equipment, like the 7600 vs the 9600, there is indeed more RAM, and that slightly helps with image quality on the DNR. Is it substantial? Not really.
There are also times where the dynamic drum is harmful.
And remember that all VCRs track slightly different, unit to unit (regardless of model). Some work poorly with some tapes. It's why many pros and serious hobbyists must own several S-VHS VCRs.
Just get whichever suggested S-VHS VCR you can afford.