The ariel is a more comfortable saddle - it's a mountain bike saddle and has padding. The ruby has a little bit of gel padding, but there really isn't much to the saddle.
I use the base model ariel on my commuter bike, it's very cush with a lot of padding, I don't wear bike shorts usually with that bike.
On my mountain bike, I use the ariel sl, it's still padded, but lighter.
i've got rubies on my road bikes. At the beginning of the season, it's a little bit hard to get my butt toughened up to the ruby.