Amateur Radio operators typically refer to themselves as Ham radio operators. There are a multitude of sites that reflect this, just to name a few:
Most search engines will translate ham/amateur radio. Better SEO should be found by calling the site ham than radio.
Of the options, Amateur Radio would be a bit of a mouthful for a URL, amateur certainly wouldn't work, radio is an okay choice, but would open the door far more to CB, FRS, and even commercial radio than most hams would really like. Overall, I think the best choice would be to use ham.stackexchange.com
Bottom line, here's pros/cons for each:
Pros- Has the word Radio in the site
Cons- Would invite more questions on CB, FRS, and other types of radio than most amateur radio operators would like. Also might draw people interested in commercial radio, which would not be on topic at all at a site for amateur radio.
Pros- Short, most amateur radio operators use the term ham interchangably, and commonly refer to themselves as hams. Ham also is somewhat used as an advanced amateur radio operator, not simply someone who has a license, but someone who knows something about the hobby beyond having a license.
Cons- Might be confusing to Search Engines. I doubt this, as a Google Search reveals.
Bottom line, I feel that ham would be a better choice than radio.