I enjoyed using the T616 to an extent. I had one for about six months as a backup phone. It didn't have great reception in fringe areas, but BT was good, the cam was okay and the menu system was average. I think SE has made dramatic improvements in its UI since then. Perhaps instead, you may consider the T637, which though a bit more expensive (I bet you could find it for about $150 on ebay) is an improved version of the T616.
I've not had the opportunity to use either of the Siemens phones you've mentioned, but the S66 can be had for a decent price now on ebay and it's practically a smartphone plus it has a 1MP cam. I'd love to try one out ... but I buy too many toys, so I have to exercise some restraint.
Now that the z520a is out, maybe you can find the z500 for a good price on ebay. Just a thought.
As far as I can tell, the only difference between the 6620 and the 7610 is design and camera with the 7610 having a MP cam on it. So, that's probably just going to be a personal preference choice. I know you can find 6620's for a decent price on ebay now, too. For six months use, it may not be a bad way to go.
Hope that helps a bit.