Go with the vertical, to heck with the G5RV.

The vertical (did you get the HF6V, or some other model) ought to work fairly decently for you, even without much in the way of radials. What radials will do is lower your radiation pattern, and could affect your impedance some. I'm assuming you've got some sort of antenna tuner, though.

If you're really limited for space, the Cushcraft R-7000 series works well without any radials at all.

To get any sort of dipole to work as well as a vertical, you're going to want to have it very long (you want to at least be able to work 40 meters), and very high--Loren's antenna system nonwithstanding.

Now, here's the real kicker. What's the overall size of your lot? (length by width)? If you really want to have fun with an antenna, I highly recommend the Loop Skywire (by my good dear friend Dave Fischer, ex WØMHS). It gets out, period, and it's even hard to see unless you're really looking for the wires. I may have the .pdf of it somewhere, if you'd like to try it.

73! (and ask me about my quad someday!)