Dewhi...The "Tweeter" in the MG1.6 is just more Mylar diaphram with lighter conductors on it. Same as the Woofer except that the conductors are metal foil rather than wires and (I think) the Mylar is thinner.
In the "R" models, the tweeter is a completely different device, a ribbon transducer. Ribbons are very good, but some think that their sonic characteristics differ too much from the Maggie woofer panels.
If you don't hear a big improvement from the ribbons, save your money and stay with the MG1.6QR. The ribbons also have some reliability weaknesses that don't exist with the QR tweeters.
If you do prefer the ribbons, by all means get them. Just be sure it is because of what you hear, and not that people told you that the ribbons are better.