When using a wirebrush (on a dremel head for example), which is more abbrasive and what are the advantages of stainless and normal steel?
Stainless is typically softer due to chromium and other metals added to the alloy to give it the stainless quality. However, the difference is certainly negligible for a wire brush in this situation. The reason they sell them in stainless vs regular is that one will rust and the other will not.
Whenever I've gone to all the trouble of getting down to my valve seats I have chosen to clean them by hand with softer tools which will not risk damage to the seats. A simple pick and brush set, etc. Heavy on the elbow grease.