Here in Northern Vermont, we have a perfect contrast of the 'mega resort' vs. 'small resort' which illustrate all the reasons people pick either one or the other. Its perfect, because one resort has '2' mountains, the other has '3'... and one of the mountains is shared between the two resorts. For those of you who don't know, I'm talking about Stowe Mountain Resort and Smugglers Notch. Stowe occupies the entire east face of Mount Mansfield, the highest mountain in Vermont. it also occupies the southwest slopes of Spruce Peak, directly across the street. Smugglers' Notch occupies the north faces of Morse and Madonna Mountain, and the northeast face of Spruce Peak (which Smuggs calls Sterling, after the pond at the summit). Heck, they're so close that the Smuggs trail map shows Stowe's trails in the background.
So as far as micro climates and comparable snowfall, the argument is moot, as they are directly adjacent, and the only contrast is wind direction, which usually favors Stowe ever so slightly. Vertical? Stowe boasts 2200. Smuggs technically has 2600, although it is not continuously skiable. Regardless, they are roughly equivalent in vertical. Their skiable acreage is roughly comparable as well, Stowe again holding a very slight edge. Smuggs brags a 'triple black', Black Hole. Stowe brags Goat and Starr, which are very similar in their degree of challenge. Long story short, the actual skiing challenge is about the same. Smuggs has a highly acclaimed children's program. Stowe can lay claim to better off trail skiing, with the summit of Mansfield.
So far, everything looks pretty similar, right? Well, this is where the two roads diverge. Stowe is the epitome of an eastern mega resort. Smuggs is an almost perfect picture of a family owned small resort. Stowe has a 300+ room 4 star hotel at its base. It also has numerous luxury condos and private homes. It has gourmet restaurants both at the base and at a summit. It has a brand new high speed main quad, a gondola, and two other high speed lifts at its disposal. Of course, Stowe also has the highest lift ticket price in the East ($92 weekend or holiday). On the other side of the mountain, Smuggs has a very rustic, basic base lodge, some condos and a basic hotel. They have no high speed lifts, and nothing bigger than a double chair on the whole mountain. Their lift tickets are significantly cheaper ($66 on weekends, $70 on holidays).
So, why do some people choose Stowe over Smuggs? Like I said, they're very comparable terrain wise. Well, Stowe has a better snowmaking and grooming infrastructure. Important in the east. They open earlier, stay open later, and have better conditions throughout the season. Stowe has a better lift system. When it's -5 out, many people would much rather spend 8 minutes on the quad, or 10 minutes inside a gondola cabin, rather than spend 17 minutes riding the Madonna chair to the summit at Smuggs. Some people don't want to eat like crap when they ski. While I'm a big fan of cafeteria pizza and fries, some people want a localvore gourmet meal for their lunch (which they can get in one of the regular base lodges, no white napkin needed at Stowe). Some people want the 4 star hotel, where they pull up at the beginning of the week, and the valet not only takes care of their car, but also their skis. So the next day when they leave the hotel, their skis are handed to them 100 yards from the lift, rather than dragging them in from the lot. Some people want a spa treatment after a day of skiing. Some people want an active apres scene. Stowe has dozens of restaurants and bars, ranging from dive bars (the Matterhorn), to classy restaurant/breweries (Crop Bistro).
So why do people spend more scratch for the megaresort? Break down the term 'ski vacation'. Yes, people go for skiing. But it's also a vacation, and many people want more out of a vacation than just the skiing. Besides, in the East, most of the megaresorts have better skiing in any case.