We rented from Europcar and picked it up on ave Foch in 2012. It is just beyond the Arc de Triomphe, easy to walk to from the metro stop at the Arc with our minimum luggage, and was almost a straight shot out to the ring road (without having to circle the Arc).
We picked it up on a Sun morning, so traffic was light...the only bad thing is we got turned around by the GPS and ended up in the middle of Paris (car had integrated GPS and somehow we ended up going to the last destination of the vehicle - the GPS was speaking German and it took us a bit to change it over), but had we just followed the instructions from the folks at the rental office, we would have been fine, because we were initially heading in the correct direction and it was simple to get to the ring road. Also able to get an automatic there, if that's a concern - some smaller pick up spots may not have the selection...
Personally, I wouldn't want to pay for train tickets if I planned on renting anyways...just an extra expense (but could be a time saver, mind you).