PollyBass : I don't know the exact reason why used Rick's are cheaper than, i.e., used Fender's, but IMHO, except _VERY_ old instruments, a vintage bass is, after all, "just" a used bass.
I also think the many events that occurred at Fender (bought out by CBS in the 60's, etc.), and the many changes that were done to their models, helped having their vintage value increasing. If you look at Rick, you'll see there's no "pre-CBS" models (I mean nothing similar), and there were very few changes in their models over the years. Apart from any consideration of what change is "good" and what change is "bad", it appears that a 1971 4001 is very similar to a new 4003, except on two essential points : the neck P.U. is the not the same, and the neck is supposingly reinforced.
To your question "4001 or 4003 ?", my reply is : "I own a 4003, but I definitely prefer the 4001's". It's a matter of taste though, but you have to know that setting up a 4003 neck requires a special Rickenbacker wrench, and the operation is a bit more "subtle" than on a 4001. In fact, I never had to adjust my previous 4001's and they always played great, with low action and no buzz at all.
My 4003 is not cream, it is white (this finish has been discontinued as of Dec 31st 1999). But according to other owners' experiences, it will probably, one day or another, fade to cream. But the real cream finish at Rickenbacker was that of the 4001 Chris Squire Limited Edition. A great finish, indeed, and a great Rick too.
All the best,