For me scrubby zooming is preferred (coming from Photoshop), but it is way to slow. I have to move the pen on my tablet back and forth a few times, whereas in Photoshop it's much more sensitive and therefore a lot quicker (I don't need to lift the pen/mouse and get a very wide zoom range). Gets annoying after a while. I can see why some prefer the marquee zoom in this case as its currently quicker than scrubbing.
Photoshop uses both command+space and option+space (latter does not interfere with Spotlight) and has a dedicated setting for scrubby/marquee zooming.
First of all, wonderful job on this app. It's so promising and I'm so happy to be a part of the community. I haven't been able to integrate it into my daily workflow because it's missing some key points, and every time I use it I stumble upon a lot of those.
One small detail which actually makes a big difference on regular usage is the fact that while creating a shape or resizing it, I can't hold space bar to move it. That's a feature most vector graphics apps out there include, and it makes life a lot easier in terms of positioning elements on the screen.