The Revell 1/96-scale kit was originally issued in 1959, and represented the state of the art at that time. In many ways it's never been surpassed. In other ways it has. For producing a sheerly impressive, big model of a clipper ship, there's nothing in the plastic model industry that can compete with it. But some of the detail parts aren't up to modern standard, and everybody who's ever built it has had to wrestle with one big joint problem: the joints between the forward, middle, and after sections of the maindeck.
The molds do indeed seem to have deteriorated a bit in recent years - and quality control, at both Revell-Monogram (U.S.) and Revell Germany, isn't what it used to be. I haven't built the kit in many years, but I've read complaints recently about untoward amounts of flash, warpage, and low-quality styrene. The instructions (especially the rigging instructions) have also been "dumbed down" in recent issues.
On the other hand, if you get hold of a ten- or twenty-year-old (or older) kit, you may have trouble with the decals. And the decals in this particular kit are rather important, in that they represent the carved detail on the bow and stern.
The Imai kit is a beauty. It's now being sold by Aoshima; I haven't seen one in an Aoshima box, but I imagine the parts are identical. This kit is considerably smaller, but in many ways it's better detailed. The seams between the hull and deck planks are countersunk, the representation of the paneling on the deckhouses is better, and most of the deck fittings have a little more finesse. And the maindeck is in one piece.
The Imai/Aoshima kit quite obviously is based on the excellent set of plans drawn by George Campbell. (Those plans, incidentally, are among the biggest bargains in ship modeling; they're available by mail from the ship's gift shop - which seems to be functioning at the moment despite the big restoration project that's currently in progress: http://www.cuttysark.org.uk/index.cfm?fa=contentShop.productDetails&productId=40&startrow=1&directoryId=6 ) . Mr. Campbell was the naval architect in charge of the ship's reconstruction back in the late 1950s.
The Imai designers seem to have followed those drawings with remarkable care. Unfortunately the kit does contain a couple of amusing mistakes that pretty clearly can be attributed to the designers' unfamiliarity with the subject - and with the English language. We've discussed those errors, which involve the two big cargo winches and the "booby hatch" aft of the mainmast, in a couple of other Forum threads, including this one: /forums/795293/ShowPost.aspx . That particular point comes up in about the tenth post of that thread.
If I were thinking about building a Cutty Sark (heaven forbid), I'd probably start with the Imai/Aoshima kit. It's currently available through Sqadron Mail Order: http://www.squadron.com/SearchResults.asp?offset=0 . The price takes my breath away (I remember when the big Revell one cost $10.00), but there are many hours of modeling in that box.
The big Revell kit, though, certainly has the potential to be turned into a fine scale model. It apparently isn't in the catalog of either Revell of the U.S. or Revell Europe at the moment; I do hope it comes back. It's one of those classic kits that should never be allowed to disappear from the scene.
Also on that Squadron search page is another good representation of the ship: the old Airfix one. It also has been around for quite a few years. Bearing in mind the limitations of the scale (which I suspect is somewhat smaller than the 1/130 claimed in the ad), it's remarkably well detailed and accurate. (The deckhouse paneling, as I remember, is actually rendered better than it is in the big Revell kit - and so far as I know it's the only plastic Cutty Sark that represents all the bow and stern carving in three dimensions, rather than with decals.) And by current standards it's downright cheap. Certainly worth serious consideration, especially by those with limited space for displaying completed models.
Hope that helps a little. Good luck.