The bike shop is giving you a bunch of nonsense. I’d be hesitant to buy from them, which is too bad since Giant makes some nice bikes.
I’m 58 years old. I LOVE my steel bikes. For years I rode a 1997 Trek 730, which I still own, and it has a wonderful ride. It’s still a great, comfortable bike, no “metal fatigue” that I can tell. Last year I had “newbikeitis” and bought a 2010 Specialized Vita Comp with an aluminum frame. I sold it already because, although I loved the componentry on the bike, I couldn’t stand the harsh stiff ride of the aluminum frame which rattled my bones. Every time I rode that bike I would ache from head to toe.
As for an aluminum frame with shock absorbers, that’s a matter of personal preference. My husband has that set up, but I find it annoying when the bicycle bobs up and down over bumps, and that causes the bike to slow down a bit as it loses some forward momentum. With a steel frame, you don’t need shock absorbers because it gives a comfortable ride as is.
My steel Jamis Coda weighs 25 pounds. My steel Trek 730 weighs 27 pounds. Both are sized medium (I’m 5’5” tall). I can’t imagine that a steel Giant Sedona or Cypress in size small would weigh 40 pounds, that just doesn’t make sense. You could try calling the Giant customer service number and ask if they can give you an approximate weight of the bike you are considering.
That number is: 1-800-US-GIANT
As for the 26” wheel vs the 700c wheel, the 700c has a faster roll to it. My husband had a 26” wheel on his Cannondale comfort bike, and he had to pedal it harder to keep up with my faster rolling 700c wheels. He recently sold that bike and bought a new Specialized Crosstrail bike, which is comparable to the Giant Cypress, with 700c wheels, and he’s much happier with it.