The assertion that you need over ear protectors as in ear versions can't work as well is unfounded and, in truth, I've no idea where this idea came from.
Both over ear, and in ear, protectors can offer about the same level of protection, assuming the same effort is put into design and wearing them correctly, against noise. When you think of it logically there is nothing special about the inch of skull around your ears that is covered by over ear hearing protection, the rest of your skull remains unprotected.
A key factor in damage through industrial noise, where most of the research is done, is incorrectly fitted hearing protection or even a reluctance to wear the protection because it is uncomfortable etc. For this reason it is important that people wear the protection that they are comfortable with so that they are wearing it during the whole time they are exposed to the noise. It is also key that the protection is correctly fitted - some people don't push plugs in far enough or some wear over ear protectors over the legs of their glasses and this is where it all goes wrong.
When it comes to in ear protection then the various plugs that use expanding foam to provide a seal will always work better than the moulded type of plug - the foam expands to fill all the voids whereas the moulded type plug can't do this. Generally speaking the moulded type plug will lag about 10dB or more behind a foam plug in terms of attenuation. However, if you find the moulded plugs more comfortable and so are more likely to wear them then they may actually be safer for you.
Anyone interested in just the basic passive foam plugs would do well to try one of the trial packs provided by the Ear Plug Superstore - I've ordered from them in the past and they were always very efficient though I've no idea if that is still the case:
www.earplugstore.com