Expanders are basically the same as implants. The main difference is that expanders don't need to meet quite the same level of quality assurance as permanent implants do. Meaning, expanders are usually meant to be left for up to 6 months, with the final goal of replacing them with permanent implants.
This doesn't mean that expanders will automatically fail after 6 months. By the same token, conventional implants won't necessarily fail after exactly 10 years.
If an expander fails after 3 months, there's a pretty good chance that you'll get a free expander (but check with your surgeon on this detail). If the expander fails after a year, you might not be quite so fortunate in getting a free replacement. There's also the question of who pays for the surgery, so be sure to get that information clear before your surgery. No "oh gee I didn't know that" later on, you know?
That said, I've heard more than once previously that the expanders that Dr Foster places are a) anatomical, and b) after the initial fill, they have a rather odd, triangular shape. The triangular shape tends to upset patients, but this is only temporary, as it will take on a more natural shape as the fills get added.
More info on round vs anatomicals, here:
http://www.largerimplants.info/joom/en/reference/60-implants/139-anatomical-vs-round-implants.html