Ive owned Standard Schnauzers, Mini Schnauzers and Shih Tzus myself....here's a run down of my experience;)
Mini Schnauzer- VERY intelligent. Almost too intelligent. With that intelligence comes a tenancy to become bored easily and get in to trouble. My mini schnauzer, Baron, loved playing in the water, and if I was taking a bath or shower I had to lock the bathroom door because when he heard the water running he could run at the door grip the door knob with his claws and turn it while kicking the door with his back legs and open it. It usually took him a few tries, so you'd usually hear *wimper wimper* *click* BANG! *click* BANG! *click click* BANG! SPLASH!. Even though Baron and Tasha were both great dogs, I have seen other people that get mini schnauzers that dont know how to train them properly (they can be VERY stubborn dogs, it goes along with the intelligence) and the dog is a holy terror. Biting neighbors, running away from their owners, yapping incesantly at passersby. They are great dogs, if you know what you are dealing with.
Standards- seem a little easier to handle than minis. they seem (in my experience) more laid back and less hyper than minis (my schnauzer, Chelsea's favorite past time is laying in my lap on the couch and watching tv). They are still very smart, and tend to get in to trouble because of it if they arent challenged. My way of challenging Chelsea is teaching her a new trick every month and playing her favorite game called "Go find it!" (we hide a treat in the house, or a toy in the yard and tell her to go find it. We used to be able to hide them well enough that they took an hour to find, but it trained her to use her nose better, and now it takes no time at all. We've resorted to hiding toys up trees to keep her occupied). Ive owned a lot of dogs in my life and standard schnauzers are tied with shih tzus for my favorite breed. Though, in my experience, males tend to be a little territorial and sometimes dog aggressive.
Shih tzu-- All of my shih tzus can be called snobby, but they are great dogs. They are smart, but a little bit more of a "what's in it for me?" attitude than my schnauzers. I can tell my schnauzers "Jump and you get a cookie!" and they say "YAY! A cookie!!" and jump. My shih tzus lift their head up from their pillows and say "what kind of cookie? Is it worth getting up for?". My shih tzus though, are little dolls. They want their hair messed with and brushed. They want to be told how beautiful they are and they want someone to carry them everywhere they go. In my experience, though, shih tzus tend to be one person dogs. My schnauzer loves everyone in the house and regards each of us as an alpha. My (well, no, my dad's) shih tzus have a different person that they adopted, not the other way around. Snickers, even though I bought him and took care of him as a puppy, claimed my dad. He loves me still, but if my dad and I say "Come here Snicks" at the same time, it is no contest, dad would win. Mitzie claimed my mom even though she was supposed to be my dad's dog, and my moms shih tzu Bonnie claimed my grandma. Shih tzus very much pick their owners.
really, if you want a dog to play with constantly, go with a mini or aussie, if you want something to cuddle and pamper and snuggle, go with a shih tzu, if you want something in the middle, go with a standard;)