Definitely not Vossens and definitely not 20". I would suggest something wider and at least equal quality to stock... Otherwise what's the point? Since your car is an '08 I assume that you have the forged 19" OEM wheels. Neither of the options you listed are equal in quality to your stock wheels. If you go wide then the OEM tires will be too stretched. I would also suggest HRE, Volk, BBS or Advan. All of those brands are out of your range new but there are tons of used wheels for sale in this forum. I would much prefer a quality used wheel over a mid-level new wheel. Best of Luck.
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'04 E46 M3 - Jet Black - BBS E88, KW Clubsports, Brembo BBK, SuperSprint Exhaust, Recaro
'15 i3 - Laurel Gray - Why would I mod this thing?