The heavier XC90 will be more comfortable to tow with in almost every way, especially with its wider track (for improved stability) and higher vantage point (for improved visibility when maneuvering). It has a higher maximum towing capacity, and with good reason.
XC70, see page 295-296: XC70 Manual
XC90, see pages 159 and 295: XC90 Manual
The 3.2L I6 is okay, but more torque at lower revolutions is always helpful for towing. As far as FWD vs Haldex AWD goes, it's situational. If you've got three or fewer passengers, AWD can help. If you have four or more passengers, AWD actually hurts towing performance. Take a look at the towing table in your XC90 manual and you'll see what I mean. With the XC70, you can't tow enough for it to matter anyway.
If you choose the XC70 for some reason, you might want to look into the camper mirror accessory to help with your visibility.
2007 Volvo XC70 | Silver / Black | Premium & Climate Packages, Steel Cargo Barrier, Cargo Cover, Trailer Hitch, Parrot Bluetooth, USA SPEC iPod Adapter, 4-Way Pioneer Door Speakers
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