Hey, Ed-
In my racing days I had the minimal Camelback like most everyone.
When it came time to get a new one I thought the Classic would be fine, minimalist kinda like the original. Well, it doesn't really hold that much more.
Fast forward to a couple years ago and I bought the Mule on recommendation as the most popular. It did not disappoint. Enough room for pump, tools, tubes, mucho food, small first aid kit, jacket/shell, longsleeve underlayer, camera, phone, keys & little bitty pocket crap. With all that there is still some room. One reason I like the Mule is the sternum and waist straps - keeps it in place when going airborne (the Classic lacks the straps, and drives me nuts flopping around on my back).
I would only get one bigger if I needed to carry more food, H2O, clothing, armour maybe.
The Mule serves me well for all day rides.
Likely I will rarely if ever use the Classic again.
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"I tell you, We are here on earth to fart around, and don't let anybody tell you any different." - Kurt Vonnegut jr.