I prefer clips between five and ten minutes (closer to ten) and priced roughly the same.
I see shop around a lot of clip stores that have really enticing looking 15 to 20 min clips (again, priced roughly the same), but I just can't justify the purchase unless the model is just flat out amazing. Primarily because they are too expensive, take up too much hard-drive space for one clip, and after ten minutes I'm probably just going to fast forward to the happy ending anyways (assuming were talking about a footjob clip).
On the other hand the two minute clips are priced great, but too short to really get into the clip. I always feel like once it's gets going, it's all of a sudden over. I suppose that's great for guys on a tight budget or who like "popshot" type clips that Nonni does. My preference however is for more story arch and depth.