[racer]

The reason for the value of a "6" tub is the VIN number. Only 3300 or so "6"s vs 118,000 "4"s.

Some thoughts:

914s are older for starters, and collectibility often starts with age.
914s are also "simpler" than 944s.
Many 914s have rusted away. Finding a good, unmolested example is quite difficult.
914s are "open" cars and when you are buying a toys/collectibles, open cars are seen as more valuable/desirable for many than their coupe counterparts.

Most folks buying "good" 914s came of age when 914s were new. To them a 914 holds a special place in their heart (emotional) that a newer car (a 944) doesn't.

But, from a performance standpoint the 944 is the quicker car.