You mentioned two different effects of your 190s, one being cumbersome and the second, gliding. Did you buy the AMG converted car as is new or did you have AMG do the work? I ask this because if bought as is, then AMG may have applied a package system to the car which may have included shocks and springs. Do both cars have the same unladen ride height?
One thing to do is borrow calipers and measure the diameters of the coil springs, both front and rear. A small difference as .005-.007" makes a large difference in spring rate which effects handling or ride comfort. If the AMG auto has lower profile tires mounted, it also would be harsher riding but better response. Another question comes to mind, have both vehicles been the same since new or has one become more different lately as in worn shocks etc?
Overall, a car with an autmatic transmission will 'glide' better than a manual car because of the direct engine braking effects and what gear you are in. The manual car is what some drivers like because of the closer feel of the road and car, with driver input.
The two cars you own are like comparing apples to oranges and not apples to apples. One is for the days you feel aggresive while the other is for church going etc!! Enjoy the best of each ones' traits!!