I've seen and tried sizeup before I made the switch, and though I feel that it's a great application and a great step towards a middle ground for tiling, it is most definitely not the same thing.

Being able to have windows automatically be positioned is key to the tiling wm, I feel. It allows you to have a layout work as you'd expect without fumbling or thinking about it.

I do agree about the inaccessibility of desktop Linux (though it seems to be getting so much better these days!). A previous co-worker of mine uses AwesomeWM every day inside of a VMWare instance with Mac hotkeys disabled and loves doing that. Might want to give that a shot if you like what SizeUp gives you.