Down Side of the FN FS2000:
No rail accessory system is currently offered beyond the picatinny rail on top of the rifle. There are foregrips that have lights, but they are to hard to find to purchase and very expensive.
The MAG release on the FN FS2000 sucks. You have to pull very hard on the mag to have it release.
The thickness of the FN FS2000 requires that some users really crane their necks to use sights whether going with the iron ones or after market optics.
The dust cover on the forward ejection of the FN FS2000 is a thin plastic and you can bend it with your fingers very easily. Doubt it would last too long under great amounts of stress.
The barrel of the FN FS2000 has some wobble when fired. Although it has not yet proven to effect the accuracy or function of the weapon.
There have been many complaints about polymer not being as robust as the AUG by many who own both.
The Flashhider on the FN FS2000 is fixed and can not be removed. It takes great expense and time to change the barrel and add a flashhider.
The trigger is heavy. Although so is the AUG at least there is an after market "trigger tammer" that brings the AUG's pull down to around 5 lbs.
Up Side to the FN FS2000
Forward ejection means that you can shoot this rifle as easily weak sided as you can strong. In other words if your right side is injured you can switch to the left side. In addition, some like to practice the weak side switch when taking corner in CQB, but those who instruct for a living have dismissed that practice and the USMC no longer teaches it.
It is lighter than the AUG.
It is like the AUG well balanced and you could shoot the rifle one handed if you needed to.
Currently the FN FS2000 is available for purchase brand new whereas currently all AUGs are either all together used, old and never used (NIB), or old with newer parts that are for sale. The AUG also currently is much more expensive than the FN FS2000. However, two American companies have announced plans to release American built AUGs and there have been rumors that Steyr Arms may be able to release the AUG in some form next year here for sale.
The FN FS2000 is not combat tested and as of now I am only aware of one military adopting it as their official infantry rifle. Of course, the weapon has only been around for less than seven years.
Down Side to the AUG:
Although the AUG can be configured for left handed shooters it can not be switched as easily from strong to weak side as the FN FS2000. Of course, as stated many believe that shooting weak side is so ineffective in terms of accuracy and in CQB situation (where switching would require taking off your tactical sling) that it should be avoided. You can always fire the AUG one handed with your weak side if you needed suppression fire to retreat to safety rather than shoulder firing it as chances are you wont hit much even with a weak sided shouldered weapon.
Magazines and parts are expensive right now for the AUG.
Right now AUGs are very expensive going from $2000 for a USR to as high as $7000 for an AUG A2.
The iron sights on the AUG suck IMHO.
The Up Side to the AUG:
Rail systems are available for the AUG from B&T and will soon be made available by American producers. However, the B&T rails are expensive, but you can put better iron sights on them, which is a huge plus.
The AUG has a Last Round Bolt Hold Open meaning that when you fire the last round the bolt stay open alerting you to the fact that you're out of ammo and making it faster to reload. A lot of folks like that feature.
The AUG is fielded by a long list of Western Nations and is combat proven in every type of environment on earth. It isn’t going out on a limb to say that the AUG is the most reliable and successful bullpup to date.
The AUG has a wide trigger guard that allows the whole hand to fit within and makes it much easier to shoot with winter or protective gloves on.
The AUG's 1.5x scope is so tough they have run trucks over them to prove this and they were still 100%. Probably one of the toughest optics out there and it does not lose its zero even after heavy and sustained fire.
There were three members of Steyr Mannlicher that were credited for design of the AUG and at least one of them has gone on to design the HK G-36. I have no idea who the lead designers on the FN FS2000 project are and what they have done in the past or are doing now.
Again...American companies are claiming that they will produce American AUGs here in the United States. This will make the purchase of parts, accessories, and legal issues much easier to deal with for the consumer. Plus, you'll be supporting an American company that provides American jobs while having an awesome rifle...can't beat that.
The AUG A1 and A2 have quick change barrels and the USR can be configured to have a quick change barrel. That means in a matter of seconds you can switch out barrels for whatever length you may want.
The AUG mags are some of the best magazines I've ever used. They have a reputation for being reliable and I've never seen any complaint about a failure caused by an AUG mag or experienced one myself.
Now I’ll admit I love the AUG and therefore may be bias. I may also be missing some things about the FN FS2000 or even the AUG, but I’m sure other folks will add some information for you. I think both are awesome weapons and you should get whatever you feel most comfortable buying. Firearms are like gloves there is no one size fits all.