Let me preface; I am well aware that Mike & Tony's is a Pittsburgh legend when it comes to gyros. I am a huge fan of gyros, and often go to M&T's downtown. However, when in Southside, when choosing between M&T & Lesvos...I have to go Lesvos. They're as authentic as you're going to get in Pittsburgh.
The owners are off the boat Greek. They're friendly, and will talk to you about anything & everything if you'd like; I usually sit at the "bar" and chat. They do a special that includes a gyro, fries & a drink for something like $7 or less.
Here's how the competition between the two stacks up -
Meat - Lesvos - I've only had chicken, but Lesvos wins. It's well marinated, and freshly grilled. M&T tends to char theirs too much.
Pita - M&T. They toast theirs with a bit more oil/butter/unhealthy goodness.
Sauce - tie. They're different, but I can't choose a winner. Lesvos is more authentic, but M&T has a sour-cream-crack quality about it.
Fries....Lesvos. No contest. They're fresh-cut, dropped when you order. M&T use frozen bagged fries. The fries are the deciding factor between the two.
I really encourage you to try Lesvos if you're in the mood for a gyro. I promise that you won't be disappointed.