Lets say you buy a new m5 and it costs 90+ tax. The m6 would be around 100+ tax. If you are in the m5 range you could go for a 911s or 4s. GREAT CARS!! If you are in the m6 range I would look into a gt3. You can prob pick one up for around 120. Yes it is 20g more, but well worth it.
My friend just bought a 08 gt3rs for a grand over msrp. YES ONLY A GRAND!! You need to find dealierships in bumb fuc& states in the middle of no ware where they cant find a buyer. Do alot of research!! You can find some great deals on new cars. I remember my dad was looking at a c4s and found one new under msrp in kansas. He ended up buying a range rover, but will buy a turbo prob in a year or so.
But between the m5 and m6, I would vote for the m6. You want a fun car, and a m5 will feel more like a daily driver when you take it out on weekends.
If i was going to spend around 100g, I would buy a c2s.
Just my 2cts