I had that same debate myself. The main problem with the Hullavator is that it may raise the height of the roof rack to the point (at least on a SUV) that your vehicle may not be able to enter a parking garage, or even your own garage!. In my case, it would have prevented me from entering or using our local airport parking lot due to the #$%^&* "security" barrier at the entrance--and that is without any yak on top.
Because the cost of the Hullavator (before Thule bought out the original Talon version) was about the same as a 2-yak trailer, I finally went with the trailer. But I also don't like having to fool with a trailer, especially in the tiny "parking" areas along dirt road access in the woods to a backcountry launch site. By far the best solution to yak-hauling is one that a lot of folks around here use (including Brad of Sunjammers!) is to have a pickup truck. This allows about 3-4 different arrangements for yak hauling that are more or less stress-free. In fact, here is one of Brad's solutions using his new Toyota Tundra.
A really good arrangement with a PU is to just lash it into the bed.
Even better is the Truck Bed Xtender that also works with a SUV.
Finally, as a last resort, you can also go with a trailer. This is the Trailex SUT-250 2-yak version that can also carry 4 yaks on their sides. Works really well and is all aluminum so it tows very easily.