If your heart is set on getting a 3-speed I would say go with which ever you can get the best price on.
From time to time I've seen new and complete 3-speed trans that were in the same price range as an upgrade kit.
Don't let the fact that you haven't torn down a transmission before stop you from going the upgrade route.
The instruction sheet is very detailed and if you're looking for top end, you'll eventually find yourself tearing it down to install the high-speed gear kit (part #77065).
Forget what their ads say about top gear being 10% taller than stock, the top-end on a stock 3-speed (parts 87218 or 87220) is virtually identical to the top end that you'll get from a stock 2-speed. First gear will be lower, but top-end is pretty much unaffected because they altered the gear ratios going from the idler shaft to the output drive shaft.
There is also a complete Savage X transmission part #87257, but it appears that it too lacks the high-speed gear (I may be wrong here, but it looks like the only difference between it and 87218 is the gear case).