I'm holding off directly comparing them until Reveal announce their final pricing. They both have pros and cons, I wouldn't right now place one above the other.
There is also XRay Editor (http://mireus.com/xrayeditor/) that comes with an Xcode plugin to relatively easily convert the changes you made into code. It also has an overlay feature to be able to adapt to a mockup.
Then there's http://hierarchydetective.com/ which is OSS, still pretty rough. It support a few different hierarchies like UIKit, CALayer, cocos2d and can supposedly be easily expanded for more view hierarchy types.
Spark Inspector and Reveal are pretty similar at the moment. SI does notification tracking and also live UI updates (and rotating the 3D view in the client by device movement ;) ).
PonyDebugger is the only one that includes network traffic analysis.
I did a talk about these and more at CocoaHeads Dresden this week. Tough it hasn't been recorded and the slides are currently in German. I will translate them and post the link later.
I'm holding off directly comparing them until Reveal announce their final pricing. They both have pros and cons, I wouldn't right now place one above the other.