We test drove a couple of X5s the other day, an '07 4.8i with sport package (and Adaptive Drive) and a '10 30i. In driving both back-to-back, I know I want the V8. Just need to do a diesel/V8 comparo now. Otherwise, both cars were very nice to drive. They seem very liveable day-to-day as well as long trips. The sport package with AD actually felt like it rode nicer than the regular suspension. We liked the updates to the '10, seemed the infotainment (aka iDrive) was easier to control. The biggest demerits we found were the high loading height and relatively small trunk. We liked it more than the Cayenne we drove but I think some more test-drives are in order.
It seems to come down to three options: 1) Buy the vehicle I'm envisioning, a nice DD that can tow an enclosed trailer the 5 times out of the year when I need it. Compromises need to be made in the way of towing capacity to get something that also works as a nice DD and vice versa. 2) Rent a truck whenever I need to tow. This is great because I don't have to buy/register/insure/repair/park anything and can DD whatever I want but it'll be less convenient and cost ~$1500/yr. 3) Buy a shitty old truck as a dedicated hauler. It's more convenient and I can use it for other errands. It will also be amortized within 3-4 years when compared to renting *assuming no costly repairs*. It will also cost extra to register/insure, require maintenance, take a parking space, and will take away from the trailer budget.