I've used all three brakes, and put them in this order (first being my favourite) MT6 - MT7 - MT5.
The MT6 offers slightly more pad clearance so doesn't rub as much, the brake holds really well (not quite on par with the MT7 / MT5, but more than enough) and from my experience, has more modulation than the 4pot brakes. I haven't checked, but would hazard a guess the MT6 brakes are slightly lighter too...
The MT7 has a little more adjustment capability, so works a tad better than the MT5 (in terms of clearance). Also, when using the 4 pot brake, the MT5 has a two pad configuration (vs. 4 pad configuration on the 7), in my experience the two pad configuration feels more reliable / has less pad rock.
If you can get one of the newer 1-finger levers then the brakes feel more 'solid' than the two finger levers (and more comfortable). In terms of pad choice, the Magura standard pads are excellent once worn in (which should take less than 30 mins), offer great modulation vs. power. I would highly recommend keeping them and giving them a try before you go straight out and buy some aftermarket pads.
Finally, the price, MT6 is middle of the road (£130ish), with the 5 being the cheapest (£90ish) / 7 most expensive (£170ish).