Too much HK hate on this board for you to get a large number of good, realistic answers.
I had an HK93A3 for a while. Excellent rifle with an excellent build quality with superb attention to detail. I love HK diopter sights. Very accurate rifle; mine had 1:12 twist rate. The military counterpart, the HK33, was not widely accepted for Military service. Off the top of my head, Malaysia used them for Military issue, and maybe one or two other small countries. As a result, the 33/93 was stifled in development compared to the AR15. Once George Bush Sr. banned their importation, the coffin was sealed.
There are lots of real 93s out there, as well as some clone builds, usually made from Malaysian kits. They are great rifles in their own right, but the AR15 design is much more efficient when 5.56mm rounds are the fodder.
Modularity/Versatility: The HK and AR are equal in this regard, a reality which most aren't aware of. Multiple forend options exist (slim, wide, picatinny, polymer, wood), the grip frame can easily be switched to allow different modes of fire as well as different grips (finger groove, thumb shelf, ambi, even one that accepts AR grips & trigger guards). Lots of stock options exist for the HK as well; A2 style fixed, A3 style collapsible, B&T style folders, and adapters that allow you to use AR receiver extensions and any relevant stock.
Accuracy: I found the HK93 to be just as accurate as a quality 16" AR with a non-Match barrel. The HK barrels are cold hammer forged and nitrided; this predates that particular AR trend by several decades. The bore on my particular rifle seemed on the tight end. Pushing a .22 cal bore brush through required more force than on any of my other 5.56 rifles.
Parts Availability: Parts are out there, but they are very expensive. There simply isn't a huge supply of German HK33/93 parts available as there are Mil-Spec AR components. There's Malaysian contract parts, and there's also RCM parts, which I've had great luck with in my MP5.
Mags: Steel HK33/93 mags are what AR mags want to be when they grow up. These mags are as tough as AK mags. There's also aluminum mags and polymer mags. I never tried aluminum, but the polymer MKE mags were very stout. Promag makes some; buyer beware. New in Wrapper German mags are around $70 each. When I bought the MKE mags they were around $20 each. I understand that they have not been available for a while now.
Weight: The 93 is a heavy beast. In its "as imported" for factor, you have a steel receiver and a steel grip frame. The A3 stock is also made from steel. The rifle weighs over 8 lbs. The AR is a featherweight by comparison, unless you've built it as a heavy pig.
Reliability: The HK roller locked design is very reliable and it runs extremely smooth in 5.56 NATO. I've also shot 91s that had soft recoil, but that is usually an exception rather than the rule.
Cleaning & Maintenance: The AR is MUCH, MUCH easier and quicker to clean and maintain. The roller-delayed blowback action dumps tons of carbon into the trunion. Cleaning out this carbon buildup can be quite a chore. Taking an industrial approach makes more sense from a financial standpoint; I used a liberal amount of Mineral Spirits to clean the trunion. Simply put, anyone who complains about cleaning an AR obviously has zero HK experience. Despite all the filth, the HKs run and run.
Cost: 2 years ago I had ~$2500 in my HK93A3. This was for an excellent condition shooter, not a collectable safe queen. I see that nowadays they are upwards of $3500 for one that is a shooter in VG+ condition. Just for perspective's sake, you can easily get a top quality AR15 with a high end free float rail, Geiselle trigger, your stock of choice and high end optics (with back up irons) for the cost of the stripped/basic HK93. You could also get two no-frills Mil-Spec quality ARs with Aimpoints/EoTechs.
For me, the AR15 is a much more efficient package for delivering the 5.56 NATO. Parts, mags and knowledge are everywhere. It's all relatively cheap as well. My AR15s have always been extremely reliable and compared to the HK93, they are easy to clean/maintain and they are infinitely more sustainable moving into the future. I traded off my 93 for a Steyr AUG A3 + $500 cash, which went towards an ACOG for the top of it. I've never regretted that swap.
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