17.8" is the way to go if you want a general infantry rifle - the 21" is too heavy. If you need shoot iron, yes, longer sight radius yada yada....but iron sight is yester-aeon for a practical/fighting rifle. We are talking about 1lb here.
The 8.9" is a bit too specialized for what it is - and that the durability is sometimes questionable. Hence, the 553 gas system.
I will pick the 551 gas system with both the 14.25 and the 17.8 barrel options, but with the 553 system in the work, I would think the 14.25 barrel 551 system is getting redundant. Remember- the longer the gas system, the heavier it is. Also, the rail system is heavy too. Also, on the book it is 14.25", but after taking the flash suppressor off, it is really a 12.5" barrel.
Therefore, a CQB with 553 gas system and the 552 LB barrel in the 12" length is going to be lighter and do the same job as the carbine with 551 barrel.