Don't get me wrong the rs6 is an excellent car. I was just joking about it being old, circa 2003.
Now I don't have an rs6 but read about them as much as I can. While both engines are 4.2 L v-8 they aren't really the same engine. The v-8 in the rs4 is FSI, and the v-8 in the rs6 is turboed so the internals and much about them are different.
that being said the rs6 engine is great for upgrading to more power due to its turbos but its tranny was designed for stock an can't handle much more.
On the other hand the rs4 engine is a highly tuned N/A engine so there is very little you can do with it mod wise but it dosen't have 2 turbos to break on it.
Summary:
RS6 - bigger car with a tip-tronic
RS4 - smaller car with a manual
If your really considering buying 1 go down to the dealer and take a test drive!
check out
www.rs6.com for a community that actually drive these(and the rs4) cars.