The HK versus the SAR difference is primarily in the lower receiver. HK made a very different lower receiver for the Model 91 so that it can't be interchanged with the G3. With the SAR, all the Greeks did was modifying the G3 lower receiver for semi-auto work.
Both guns are heavy, tipping the weight scale at around 9.75-lbs empty. Both have very heavy trigger pulls. Both have good mechanical accuracy when you can overcome the stiff trigger pull - can expect near 1-MOA with good hunting/match loads. Violent ejections that score the brass with the chamber flutings - the brass can be reused a couple of times at best before they develop splits. Extremely dirty chambers and bolt heads - hence the fluting of the chamber for better extraction, but clean everywhere else.
Rifle is not very long with the barrel just short of 18" including the flash suppressor. Abrupt recoil instead of a heavy push like the FAL.
From what I understand, the PTR has plastic lower receiver and many different options for barrel length, thickness and the same for the stock. Very reasonably priced for a newly made rifle.