I have some simple buttons in my iOS app. I'm able to draw them propperly with quartz 2d, but I could also use png-slices to build them.
What would be nicer to the iPads CPU and allocations?
If I were you I'd take the factors of resolution independence and application size into account as well. Simple UI elements even Apple recommends to draw in Quartz (See WWDC 2011 - Practical Drawing). Most of Apple's own Apps do it that way as well (Stocks App, Weather App, a good part of iPhoto etc.)
Performance wise I've ran some tests here ( https://github.com/pkluz/PKCoreTechniques ) by creating an AngryBird solely in Quartz with Bezier Curves of fairly high degrees. Now for an iPad 3 I'd have to provide fairly large images to do so (>2-3 MB just for ONE image on said device) but with Quartz it's just a couple of bytes of code. The rendering of said Angry Bird was most of the time about 15-18% slower than loading an image but I assume that'd quickly change if the pixel density increased any further. For simple shapes Quartz should most of the time be faster than loading images from the bundle. Though remember, the growth isn't linear, so depending on complexity and size those can quickly change in favor of images.
Remember how Apps doubled in size when the iPad 3 came out. You wouldn't have that problem with Quartz and your app would be ready for high-res without any additional work:)
The rule of thumb I advise people to follow is: "If I can't wrap my head around the geometric description of an object right off the bat, I will use an image."