Found this on the LI forum, reposting for reference

Conclusion: a slim head is better for extreme angle shots, but slightly worse for straighter shots and sun interference.

Quote Originally Posted by ahmadr
Quote Originally Posted by
Quote Originally Posted by lacning74
Ivan, I wonder if you could post the performance and design differences between the slim and regular head? You have stated that the slim offers better extreme angle coverage. Is that both in detection and jamming signal? What are the disadvantages of a slim head?
the regular and slim heads share the same PCB board as well as the didoe and the circuit design...etc. The only difference is the outter case, in which the regular head has a longer case. This slight extension of the case length results a "shade" effect insdie to the receving diode such that it reduces the level of sun interference some what as compare to the slim head.

Another difference is the cover lense. The slim head uses a more "convex" lens in front of the receving diode, thus the receiving angle is a bit wider. The down side of that is possible more sun interference since there is more source of noise.

Conclusion? forget what is stated above and just go for regular head for the front and slim head for the rear. Reason being there is not that much room to install for the rear, so slim is the only perfect choice. As to the rest (wider receiving angle, sun interferece...etc), dont worry about it, those are just theoretical. and seriously, the difference is hardely noticible.

myabe ivan can add more info.
Thanks a LOT for clarifying this. Since they are using the same board, let me chime in:
The slim head has less room between the lens and the sensor. Therefore, even with the more convex lens, it has a larger field of view. A larger field of view means less amplification, therefore less sensitivity (but in a wider field of view)

So, all in all, a slim head is better for EXTREME angle shots, but slightly worse for straighter shots and sun interference.