It really depends on your cable signal strength. I am fortunate mine is strong and cable is on par with an average dvd transfer.
In regards to stretching - after 20 minutes of full screen glory you'll wonder why they even bothered including the 4:3 mode with gray bars.
At the absolute least you have to get HBO to enjoy the HD on 509.
If you are a first time subscriber, you should get everything for at least a month so that you can settle into a programming plan after sampling it all.
If I remember right you can change as frequently as you like, so enjoy!
I too, own the sd582 so I am a little biased.
I got this thing in December and had also narrowed it to the Tosh and Pioneer. I was fortunate enough to be able to do a side by side at a local retailer.
I personally thought that the Pioneer's doubler did a better job with cable, while the Tosh...