#1 Bullpups are a newer design and allow for longer barrels in the same length rifle, therefore they deliver more lethality in a potentially shorter package. Given the benefits of a shorter package in CQB combat combined with the increased lethality of a longer barrel, Bullpups are clearly better. If you're not playing favorites for national pride or nostalgia, it's cold, hard math - Bullpups are the best type of weapon system.
#2 AR15s are used by the greatest military force in the world - and the soldiers overwhelmingly love the M4 and M16 variants of the AR15 that they use. There is no more diverse or well-engineered aftermarket for any other gun in the world - the options and the variety of the AR15 platform are unparallelled. It can be built to fit any shooter, having adjustable stocks, different handguard lengths, lightweight options, a large variety of grips to fit virtually any hand, etc. Given its ability to adapt so well to any situation with all the options it has at it's disposal, the AR15 is clearly the best rifle in the world.
They both sound like compelling arguments, right?
So, you may be asking, "Which is really better?" The answer to that is not a simple one. I have to qualify the answer I give. Today, based on the currently available Bullpups in the market and the cost of the AR and its variety of aftermarket parts choices, it's the AR. It's not even close. But, it should be close. In fact, a Bullpup should be better than the AR... but, it's not.
So, why is the AR better? Let's look at the Bullpups out there that can compete with an AR, so we can compare.
The problem is that virtually every Bullpup is proprietary by design. What do I mean by that? Well, you can't switch out parts on an FS2000 if the whole gun is one big tuba-like piece. You can't replace any parts because they're all one part, so they're not exactly modular. Same with the Tavor. Kel-Tec didn't build much modularity into the RDB, but the options there should at least be pretty reasonable cost. Welcome to the world of Bullpups - they are designed to be all one piece to maximize sales for the company that created them. Basically, they're making throw-away guns for militaries instead of making modular guns that people can customize or improve upon in the aftermarket. These manufacturers are making the single biggest mistake they can make: they're literally trying to cut out the aftermarket. BIG mistake! The aftermarket is what has kept the AR15 up to date and viable today. It's what drives the custom building AR market. And, they're cutting out the aftermarket intentionally to maximize profits... or, so they believe. The aftermarket is what makes the AR the best rifle available today and it is what is going to determine the next great rifle. By cutting that out with their proprietary designs, they basically ruin the civilian market for the gun and the weight of these single-piece polymer monstrosities ruins the military market.
The thing is, Bullpups should dominate the market. The problem is that manufacturers are looking for profits in a short-sighted way. They keep producing new, proprietary designs that are destined for the scrap heap. Until the rifle manufacturers, or at least one excellent manufacturer, produces a modular design that doesn't cost an arm and a leg, the AR15 is going remain the king. If that manufacturer receives a US military contract, you'll see a paradigm shift in the market. Having that kind of financial backing will lower production costs and create a military market for parts and that will trickle down to the civilian world. That will be the only way to dethrone the AR. Until that day, stick with an AR - you don't want to be stuck holding the bag on a discontinued rifle that no one wants.