One ICSP header is used to program the 16u2, you need it to load the FW and fix the FW if it gets corrupted somehow.
From posts on the forums, it seems like the 16u2 is more fragile electrically (how often does someone using a clone board report having damaged the CH340G or CP2102? I think I remember ONE post like that in the past 2 years. How often does someone report blowing the 16u2? A couple times a week). This makes me hesitant to use 16u2's as serial adapter in any design I might make. The CH340G is easy to handle as a part (SOIC-16 is easy to solder), easy to make custom boards for, and about 25 cents a pop; I think the pricing on CP2102 is similar.
I would never use an FT232, as I am not equipped to differentiate between real and fake units.
The FT232RL has an internal oscillator, instead of using an external crystal; this is an advantage in terms of reducing external component count.