I've heard mixed things about Turtleshack as well, some good and some bad. But, to say there "only going to last a week" is a bit extreme I think...
Depending on what sex they are, you may or may not have to upgrade to a larger aquarium in the future. Males tend to stay pretty small and only get to around 5". If you happen to get a female or two though they can get about double that size, in which they would definitely outgrow a 40gal and need something bigger.
Here's a good caresheet on them:
http://www.austinsturtlepage.com/Care/caresheet-mississippi_map.htm
Good luck with them!