Posted on December 11, 2012 at 01:44I think they are more robust commercial solutions, with better support, updates, and usable with many ARM targets. The U-Link's are tied to Keil tools, the J-Link's are supported by most tool chains and can operate in a stand-alone fashion.
The smaller header takes less space so in many cases will be preferable, but is less mechanically robust, something to consider if you expect it to be used a lot, or roughly.
The 10-pin connector is becoming more popular, expect more pods to support it, I'm using an adapter that ran perhaps $7, Keil U-Link's have a header you can populate, and some of the U-Link ME adapters come with connectors and cables.
http://www.keil.com/ulinkme/
The 20-pin header has been popular through the ARM9 era, figure about a decade or more, and prior to that the ARM7 designs often used a 14-pin header.
I some situations you can choose not to populate the headers, I've seen a couple of designs which just have surface mount land patterns, some times on the back of the PCB, and which can be used as probe points in final test.
