How long does it take to download?
Download speed will depend on your internet connection, which unless you have a problem with your wireless network will be the bottleneck. See:
http://support.apple.com/kb/HT1577?viewlocale=en_US
for representative download times.
Is it better to download to ITunes then sync the Touch?
That's probably the way I'd do it. My understanding is that you can buy movies directly from the iPod, but I'm not certain you can even do the rental from the iPod itself, though it's possible that you can. Either way I'd still probably do it from iTunes on the computer and then transfer it to the iPod.
Anything else I should know about renting movies from ITunes?
The primary thing to keep in mind is that you have 30 days from purchase to begin watching the movie, and 24 hours to complete watching it once you begin. And the procedure for transferring a rental to the iPod will be different from synching music and purchased movies. See:
http://support.apple.com/kb/HT1390
Regards.