You'll for a start get the 7g plus box, which is supposedly an improvement over the 7g box. Thats probably a bit like saying a turd after a roast beef dinner doesn't smell as bad as one after a curry, but really they both stink as they are both essentially the same, turds.
Back to back stats of cars with 5g and 7g plus (BE E/C 220 and 250cdi) reveal that the 5g cars have appreciably faster acceleration and appreciable worse fuel consumption. However, I'd rather have the faster car. I drove a BE 212 5g E220cdi and it was marvellous, fast changing, smooth, always in the right gear, and the gears were well spaced for fast driving with no interuptions/changes at the crucial 50-75mph speed range.
7g cars in my experience of owning one and driving numerous other ones, jolt, buck, change gear slower and have an annoying habit of needing a change around 70mph meaning fast A and B road driving has an interupting gear change which affects the balance of the car and its overtaking performance.
A twin turbo diesel provides a strong amount of torque from very low down in the rev range, right the way to nearly peak power. 5g makes good use of this with reasonably broadly spaced gears. 7g and its sister box, have 4 tight gears from 1 to 4, 5th will take you beyond 110mph, and 6th and 7th just for overdrive essentially.
One plus, 7g does allow very high speed gearing, over 150mph in my car, and 1800rpm at 70mph with a 39mph/1000rpm ratio. You get a whole 200rpm lower from a 7g box over a 5g box at 70mph. It does make for good MPG.
IIRC the final drive will be taller in the newer cars, so expect 1600rpm. In a 220cdi, I would envisage frequent and irritating down changes on motorways and A road driving as it struggles with any gradient presented to it. In a 5g car, you will not experience this issue and enjoy seamless and smooth comfort as well as a better performing car. PS, before the lynch mob come, I have heard somewhere of really bad jolts from a 7g plus car. Its not to late to keep your own, vastly superior car.
So in a few weeks, you can answer a question for me. Has Mercedes perfected the art of polishing a turd, or does the foul smell still linger? You tell me, I have told you my thoughts.
Arguably, if a customer is happy with the way a car, especially an automatic, feels to drive, it doesn't matter whether it has four gears, seven or seventeen. 
