I've used an Ol' Man climber and a Summit Viper climber.

The Ol' Man, I guess, can be used as both sit and climb or hand climb. It seems to make more sense to use it hand climb - the sit and climb position never felt right to me, but it was the first time I'd used a climber.

The Summit Viper is designed to sit and climb, but can also be used to hand climb.

After trying both (on the summit), I decided the sit and climb method, once you get used to it, is much more efficient and secure, although you can't take as large of "steps". And it doesn't take more than a minute to get used to it.

That might be exactly why it feels more secure and efficient. Kind of like riding a bike up a hill - if you're patient and spin in a lower gear, it's more efficient. If you try to push a higher gear, you might think it's going faster, but the cost is more muscles and energy needed. In the end, going slow and easy gets you up the tree quieter, less sweaty and breathing easy.

If I ever use that Ol' Man climber again, I'll definitely try to use it in the sit and climb method to see if it works better for that one too.