I personally prefer Spark Inspector over Reveal after having tried both. I find the interface much easier to read, though I do like the overall darker appearance of Reveal better. One feature that SI has over Reveal is the notification inspector, which I've found useful at times. Aside from that there's no noticeable difference in the feature list that I've run into.

Reveal does do a better job of presenting views with pattern image background colors. In SI these are often not displayed at all.

Edit: It looks like one of the release note items in 1.1.1 is "Views are now rendered more correctly in the Spark Inspector. Additional support for UIColor pattern images, CAShapeLayer, and more."

I personally do not find the 3D view in either one useful at all. It's too noisy with something of any complexity to see the hierarchy, and particularly in SI's case with limited background support it can be harder to see certain elements.

My two biggest wish list items for either one would be:

1. CALayer hierarchy inspection in the same manner as the UIView inspection works. I frequently use CALayer hierarchies when building custom controls rather than UIView ones because I find it easier.

2. Constraint visualization. It would be very useful to be able to know when custom constraints have been overridden (e.g., in a UITableViewCell subclass) while trying to debug layout problems.