I think you could go for either one.  Pretty much a wash with both of them being owned by Vail.  Both are pretty good for beginners.  Once you get off the bunny slopes, Sidewinder is a great green trail at Whitetail and Dipsy Doddle is a good one at Liberty.

Instructors at both mountains are great, you may want to think about what happens when the lesson is over. The mountains are quite different-

If your friend is athletic and picks it up pretty quick lapping the ez rider quad at Whitetail is a bit more fun and exciting, the trails have some variety without being too challenging.

Liberty's trails under Dipsy and First Class lifts are mostly flat, which is great for someone who may need to ease into it.

Whitetail also has similar flat terrain under their U-Me and Lift Off lifts, but the trails are really short.  Liberty better choice for the flat terrain. 

OrangePeels wrote:

Thanks to both of you! I'll probably choose Liberty, then, since it's slightly closer and the terrain park is pretty built up (at least it was last weekend) so I can mess around while he's doing his lessons.

 I'd honestly go to Whitetail. You can lap the high speed quad and get in quite a few runs while your friend is in lessons and for the first day I think the progression at Whitetail is better. More importantly, the green trails are a bit more isolated and you don't have a black diamond merging into beginners areas. I think for progressing from green to blue trails Liberty might be better as Whitney's is a nice trail, but for green trail progression I would start at Whitetail. The trails on the trail map pretty much go from easiest to hardest (left to right) which is a VERY nice design in my opinion.

Also... I feel like the top of the lift at Liberty for Dipsy is always icy and has a slight double fall line, whereas the lift for sidewinder at Whitetail is a bit more beginner friendly.