A 3.5 would certainly pull your trailer but if you weren't happy with the 4.4 I don't think the 3.5 will be much different.
How often and how far are you towing? If it's only a couple of times a year the 3.5 might not be so bad, but have you considered a Diesel? That would be my first choice.
Also, I would recommend getting one with a load leveling suspension if you can. It's not a must have, but when I was searching I decided to get one with and I'm really glad I did. I personally prefer to not have the rear sag at all.
Discussion about standard vs load leveling suspension here
http://www.xoutpost.com/bmw-sav-foru...uspension.html
There's a towing thread here
Towing Photos with X5. - Bimmerfest - BMW Forums