As others have said, on average the American Standards are quite a bit better than the MIM Standards. I've owned both, and while the 2010 MIM was a fantastic value and a very good bass, it really wasn't that close to my 2003 Am. Series four or (recently sold) 2008 Am. Standard five. IMO, the most important difference was the neck. The MIM wouldn't set up with action as low, and the neck wasn't nearly as stiff as the graphite-reinforced MIA necks. I also prefer the feel of the MIA necks. So, generally, if the cost isn't an issue, go for the MIA. The 2008-present models are better on average than the earlier American Series, but the latter are generally great too. My 2003 is fantastic, as was a 2004 MIA P I used to have.
However, if you can find an MIM with a fantastic-feeling neck that sets up with low action, that's a great way to go. You can easily upgrade other components, particularly pickups and bridge, to create a really great bass at low cost. I added MIA Fender pickups and a Gotoh 201 bridge to my MIM, which were very worthwhile upgrades IMO.