Planet13 - 7:53 pm on Dec 4, 2011 (gmt 0)
Based on my own personal experience I'd go with shorter URL's every time. Not just because it doesn't seem to affect the SEO... but because users appreciate it.
And better chance that users will copy your CORRECT URl when they link to your pages.
I've seen a handful of sites that link to my pages where the URL was truncated (and thus, was a broken link) because the URLs on my site were apparently too long for their system to handle...
The drawback (and I don't know how significant this is) is that people often use the URL of the destination page as the anchor text of a link. So if your URL is just domainname.com/?a=115 it would not have as much value as anchor text which has keywords in it (which a longer URL would provide).