Diagnostically, yes that is the major difference, but Aspergers, Autism, and ADHD are just symptomatic descriptors. There might be all sorts of differences that aren't captured by the symptomatic description. In the original context, I can be fairly confident that "difficulty using self control to stop doing interesting activities" is true for many people who have ADHD (for individualized values of "interesting"), but I can't make broad statements like that about autism/aspergers.

What I meant that biological basis is less understood - as in, the various mechanisms which cause the syndrome, which structures are affected, and why those structures behave differently is less well understood for autism/aspergers than for adhd. Part of this is because Autism/Aspergers effects multiple structures, which is not the case for many diseases (Parkinson's is primarily the basil ganglia, ADHD is primarily the dlPFC, sociopathy is (probably) primarily the amygdala and vmPFC, and so on)