I have a Manfrotto 680 monopod with a 3229 head for use under a Tamron 200-500mm on my 7D.
This is how I ended up with my monopod, maybe it will work for you too.
First, I looked at the stuff I already had. I have an old school Bogen/Manfrotto 3001 tripod with the grip action 3265 head (it's sort of big, like a joystick head). I love the 3265 head and plan to stick with it from now on.
The 3265 head has a QR, like most tripod heads. So, to keep my life simple I decided to get something that would share the same QR. With the same QR I don't have to swap QRs around depending on what I take with me, no QRs or tools floating around in my bag so it cuts down on stuff I might have to buy too. I leave one QR on my camera body and one on my Tamron lens and I don't have to mess with it. Ever. Nice and simple.
Now that I had the QR picked out, I only had a handfull of choices for compatible heads that use the same QR. I made the choice to use a monopod specific head, the 3229. The 3229 head only turns on one axis, so you can turn the camera on it's side for a potrait. I don't see the need for a full ball head on a monopod. In a pinch, I can turn the QR 90 degrees and bend the head backwards/forwards and use the colar on my lense to rotate for potrait mode.
Now it's time to choose the monopod. I wanted something that I could use as a walking/hiking stick. Something I could lean my weight on a little if I had too. The 680 has a pretty good rating as far as what it can support and it had a really nice, solid and thick feel to it. It's affordable so if I do something really stupid and break it I won't be too upset.
I hope this helps!
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Minolta 7d user