I agree with Akichow & SN - Shasta is more demanding than Whitney. I've done both many times, and have done each as dayhikes and also as overnights. I'd dayhike Whitney without much reservation, but as I get older I doubt I'd attempt to dayhike Shasta again, even though it's "only" 7 miles & 7K' from Bunny Flats to the summit.
Over the years I've arranged several trips for my New England hiking friends to the Sierras and Cascades. For our "warm-up" hike we'd do Whitney first, and work our way north towards Lassen, Shasta, Hood, Adams & Rainier (wouldn't necessarily do all of them on any given trip). A day or two after Whitney I'd offhandedly mention that Whitney was a good warm-up for Shasta and would get some startled looks. But, after Shasta they'd be the first to agree - it really is tougher for most people.
As for the slope below Trail Crest versus the slope below Red Banks - physically I think they're both about the same. What is different is the enormity of the slope on Shasta on a snowy year. The first time on Shasta it had been a very snowy year, and the area known as "the heart" was not particularly apparent as it had several feet of snow on it. On that first trip it was very difficult to overcome the vertigo of looking first right then left and seeing nothing but a huge, white expanses. Mostly above Helen Lake it's a head game, and remembering to snack and drink regularly. But, I digress.
