From wikipedia, just because it makes it easier explaining.
The most common non-latex condoms are made from polyurethane. Condoms may also be made from other synthetic materials, such as AT-10 resin, and most recently polyisoprene.[
Polyurethane condoms tend to be the same width and thickness as latex condoms, with most polyurethane condoms between 0.04 mm and 0.07 mm thick.[30] Polyurethane is also the material of many female condoms.
Polyurethane can be considered better than latex in several ways: it conducts heat better than latex, is not as sensitive to temperature and ultraviolet light (and so has less rigid storage requirements and a longer shelf life), can be used with oil-based lubricants, is less allergenic than latex, and does not have an odor.
Polyurethane condoms have gained FDA approval for sale in the United States as an effective method of contraception and HIV prevention, and under laboratory conditions have been shown to be just as effective as latex for these purposes.
However, polyurethane condoms are less elastic than latex ones, and may be more likely to slip or break than latex and are more expensive.
Polyisoprene is a synthetic version of natural rubber latex. While significantly more expensive, it has the advantages of latex (such as being softer and more elastic than polyurethane condoms) without the protein which is responsible for latex allergies.
So I guess if you are allergic to latex, these condoms are better than nothing.
Diaphragms do not protect against all STDs, only the ones that effect the cervix. The condom is way better.