I agree with Augee and the other members here, we're Colt guys so naturally our answer would be get the Colt.
From the other past FN threads here it seems the FN A4 rifles are not yet to the same standards as their military counterpart, but give them some time and they should catch up with Colt if they continuously improve their product.
I like the Colt AR15A4 and I'm planning to get one to go with my all Colt parts A2 clone build with USGI M16A2 upper half on Colt Match Target HBAR fixed A2 stock lower half.
I've been a Colt fan longer than you, me about 43 years now and I've had four Colt rifles in the past, my AR disease started with the SP1 in 1971, then a Sporter II (1984), A2 Sporter HBAR (1986), and a Sporter Target (1991).
With the current Walmart Colt LE6920 Magpul edition sale of $899 I'm very tempted to get this carbine first before the A4 rifle.
Here's one of the first reviews of the Colt AR15A4 rifle.
http://www.gundigest.com/firearm-gun-reviews/ar-15-review-colt-ar15a4
Heads up to all members here, you could enter a drawing to win a Colt AR15A4 rifle at GunDigest site.