Personally I think the Infinity and Ninja are going after 2 different areas in the market. The Ninja being for someone going after silence (passive) or extremely quiet (adding a slow speed fan, which there are quite a few good ones). With the spacing of the Ninja's fins, a high speed fan isn't going to show much if any improvement over a slow speed fan. It was designed for passive/low aiflow setups.
The Infinity, with its larger surface area and tighter fin spacing, seems like it will offer better cooling at a lower noise (than retail hsf or other heatsinks) for someone looking to run top of the line gear, possibly OCed as well. Though I've read a couple people getting pretty good results with an Infinity being ducted to the exhaust fan.
I like the Ninja because of it's symmetrical design. No matter what platform, if you use a fan it can be positioned to blow at the back exhaust fan. Also I'm a bit squeamish when it comes to hanging the Infinity+fan off the board. Quite a bit of weight and the mounting for AM2 I hear isn't that great.