Now, this begs the question, how does one differentiate between Eastern and Western maple?
There are a bunch (over 100, if I'm recalling correctly) of different species of maple, so simply saying eastern or western probably won't get you far.
One thing worth noting; one of the best figured maple sources in the world turns out to be China. It doesn't hit the US very often, but it might soon. It is, however, showing up in some really amazing asian-made guitars, both as veneers and as full-thickness figured maple caps.
Some manufacturers are beginning to put eastern hard rock full-thickness caps on their guitars with a veneer of western figured maple over top of that. This gives you whatever sustain benefits there are to the eastern maple while allowing you to have some really amazing figure on your top as well. The other cosmetic benefit of this is that the thinner veneer allows you to maintain a closer book match across the face of the guitar, where carving a piece of figured maple will often lose some of the book matching, especially as you begin to carve more off it to create the low areas out toward the edges of the guitar. That's the good news. The bad news is that if you get ambitious and want to refinish the guitar, you may find yourself sanding through the veneer into the plainer hard rock maple beneath.