What kind of beer for beer brats?
Since most are pre-cooked (in US Supermarkets anyway), and you are just re-heating, your (still tasty) options are pretty limited with respect to beer brats

From the perspective of UW Madison (self-proclaimed "Brat Capital of the World"):


Simmer in beer first, then grill. For more info here is a post regarding prep; this method would definitely be the preferred method for upping the intake of moisture without losing it during grilling (so long as you don't lose the brat flavor by wasting the beer you boiled them in)
For an incredibly "persnickety" method:

  I have found that if you cook the brats on the grill first, then put them into the beer they don't shrink as much.  Here's how I do it: I brown the brats lightly on both sides, then I place them in 'warm' beer for a few minutes.  Repeat several times until brats are cooked thoroughly.  Now you can put the brats into 'hot' beer to hold for individual serving...  When I said brown the brats lightly, I meant about 20 seconds per side on hot coals then into a 180 degree beer bath.  Any hotter and they will shrink.  I continue alternating the grill and beer bath until they are fully cooked.  It's a lot of work but my brats weigh almost as much cooked as when they went on the grill.  Another thing you might try is adding chopped onion to your beer bath."
  

In a non-tailgate, just getting them cooked in a hurry setting:



For grilling, I put them whole on the rack, otherwise you will lose moisture. Adding beer (while grilling) won't really do anything but cool them and the charcoal down; but you could make a beer-mustard
For stovetop, I barely cover the brats in beer (typically watery domestic like Miller High Life) in a pot/pan that can accomodate the quantity of brats being cooked and heat until beer reduces like a glaze. Wait until casing browns up a bit, then enjoy. In this instance I fork them a few times to absorb beer.
For microwave, same as stove basically, fork and nuke covered with beer.


As for type of beer, I think watery domestic does it best for the boiling since the carbonation is high and the sugars glaze up nicely before the brat is over-done. For the beer mustard pale ales are recommended by some, but I like brown, spicy mustard and might try something sweeter like a porter and banging heads with some punch from horseradish.