By studying the Terra Cotta army of the Qin, archeologists are able to ascertain that the Qin Army also uses formations similar to Macedonian Phalanx, that using 18 to 21" pikes.
With matrices of pikemen and crossbows, the Qin Army resembles more like the Swiss army in the 16th Century.
Many of the comments so far assume a fairly comparable supply situation and equally skilled commanders. While this is fun because it forces a focus on hardware, decisive factors would include terrain, leadership, intelligence gathering, who was invading whom, logistics and support of the local population.
Let us assume for this that Alexander was invading and the Qin were defending (at least initially). Most forum participants seem to unconsciously take that line anyway. Next let us get a list of the sorts of commanders that the Qin would field against Alexander and let us assume the first commander the Qin field fails against Alexander before the Qin tap their best talent against the legendary western commander. Until we know more about who Alexander would be fighting (once the Qin understood the true magnitude of the threat). Then we could talk about the differing known psychology of Alexander vs ??? of the Qin.
One question I would have is the state of Chinese military rocketry at the time. While not very effective in terms of destruction, if invented by then the Qin would have an excellent means of disordering Alexanders best cavalry units. This was similar to the problems Alexanders army had with those featuring war elephants. (elephants apparently scare the heck out of horses, according to historical records).
As for terrain China's is very flat and open on the whole, but is it so in the region that Alexander would most likely enter from? The Chinese would initially have an advantage over knowledge of terrain and local support, but not for a very long time. Alexander was extremely adept at the politics of "winning hearts and minds" in a way no modern army or commander has ever been. (a possible exception of Douglas MacArthur administering postwar Japan which in any case was much smaller then Alexanders Persian domains).
For this discussion to really sizzle we should determine the most likely scenarios under which ancient east and west would fight. Would the first Emperior of the Qin more likely be in his prime during Alexanders invasion or be weeks from deaths door by the time Alexander made it to as far India and China. Can we assume that if Alexander drove for China first he might have avoided the disease that weakened him in his abortive India invasion?
I hope this post inspires additional research and a very through discussion as I think it just might.
