Shell guy, while this seems like an easy question, without knowing you, your riding preferances, comfort level, etc, it's a tough.
Which do you like better, blondes or redheads?
Kinda the same question. Here are my thoughts.
If you haven't bought already, hold off, rent each sled for a couple days and see what you think then. As RTR stated, it's really a coin toss. Another consideration is how often you want to replace your sled? If you want to get a new sled every couple years, it lessens the impact of buying the older Cat, cause it's a great sled to learn on and you would be upgrading soon anyway.
I can add this though. . .
The Cat is a great sled. It is a little older technology, but arguably still one of the best sleds on the snow. Sidehilling wonder, strong tough motor, rock solid platform. One advantage to the Cat would be the adjustable bar. If you don't already know how you like your bar, that's the sled to decide on, as you can adjust the reach with a simple pull/push. NOT a great sled on the trail. Although some dedicated Cat guys may be able to help you work that some. Again, sweet use of a rental. Also, Cat just came out with a new sled, so there are probably going to be some great deals on 1 or 2 years old sleds. And the M sleds are awesome.
The Poo is much newer technology and is highly reguarded as a great mountain sled. You wouldn't have the excuse of having the old sled in the group . . .
. It would be a great sled to own for several years as it's in the middle of the technology progression right now. You would have a great sled to learn mountain riding on as it would be up to anything you through at it. The Pro's introduction last year has led to a large influx of used 1 or 2 year old Poo's also. I believe many would tell you that 9/10/11 Cat's are better than 9/10 Poo's.
So, that is about all I know to tell you. If you have any specific questions I'd be happy to try and answer them, but honestly there are folks on here that are much more experiance than me. You are going to have a tough time though getting people to bite on your thread title. Maybe try a differant angle and ask specifically what folks like about the sled they are currently riding.
The folks on here who's opinion is worth listening to, are going to be reluctant to get involved in a "best sled" war.
Good Luck and please post what you decide on and why!
Bag