I would not use your 20" rims for winter if you can help it, unless you only ever travel in cities and nice motorways. Read the MB manual and see what it recommends for your AMG. It will probably suggest narrower tires on smaller rims. This is the sum total of my experience on the subject (for my GLA250) > http://www.sinclairdesign.com/b/blog/piste/winter-wheels-and-tyres You are very wise, by the way, to consider fitting winter boots.
Manual states 18s will fit over the brake calliper, but I think I will go 19s have 20s with summers.... opponeo.co.uk have some good tyre deals on veredstein, have had in the past from Germany, 19s at a better price than for 18s. Offset or ET is important for our wheels unless you use spacers,
Check out ebay.de for oem Mercedes wheels with winter tyres... 18"looking for 19"and as the car gla is relavtively new to the market more oem wheels will be cheaper in the next 12 months.. that's if youbuy winter wheels as spare set a lot cheaper than from the $tealers lol