HK ships G3s, MP5s, HK-33s, etc, with both the flapper and button mag releases.
The reason it was eliminated on HK-91s and their non-nfa kin is that the flapper pivot on the same pin/tube that normally serves as the front push-pin receptical. The presence of that hole makes the receiver a machinegun in the ATF's eyes.
BTW, the flapper simply actuates the push-button. So, you really can't have a flapper without the push-button.
As said, the flapper is much easier to manipulate than the push-button. With the flapper, you simply grab the magazine and your thumb tends to fall on the flapper naturally.
As for the desire factor, to install a flapper on a HK-91, you have to drill the receiver's clip-on ledge. If you do it improperly, you've made a machinegun. If you do it properly (weld over the blind hole), you'll most likely need to refinish the receiver. Most people have a HK-oriented gunsmith do the job, so you're talking money and time put into it that a run of the mill push-button HK doesn't.
When I had my Portuguese HK built up, I had the flapper installed. It cost an extra $100 to do it with the build and it was worth it. With the clipped and pinned plastic SEF frame, it looks just like a G3.