There is more than one type of expanding ammunition. The two most common types are Hollow-Point and Jacketed Soft-Point.
Hollow-Point bullets have a hollow cavity in the nose of the the bullet. Upon impacting the target, these bullets rely on the hydralic pressure inside the cavity to force them to expand. Hollow-Points most commonly have a lead core and copper jacket although all-lead HP's, all-copper HP's, and HP's with lead cores and jackets made of other materials like brass or aluminum are not uncommon. Hollow-point bullets typically expand the most aggressively, but also penetrate the least in soft tissue although most modern premium HP ammo is still able to penetrate at least 12 inches which is considered by many to be the minimum depth for self-defense against human aggressors. Hollow-Points are the type of ammunition most often recommended for defense against human aggressors.
Jacketed Soft-Points do not have a hollow cavity. Instead, they have a soft lead core and a harder jacket made of copper, brass, or other such metal. Unlike FMJ ammo, JSP's have the soft lead core exposed at the nose and rely on deformation of that core upon impact with the target for their expansion. JSP's penetrate much deeper than HP's normally do, but they rarely expand nearly as much. JSP's are most commonly recommended for hunting big game (whitetail deer and larger) due to their deeper penetration than HP's or for practice due to their typically lower cost.
From the tone of your post, it sounds as though your main area of concern is defense against human aggressors. This means that you're probably most concerned with the difference in performance between HP's and FMJ's or other non-expanding bullets. The answer to the question is so long as the HP ammo in question still penetrates adequately it is more effective than FMJ although the difference is unquantifiable due to many uncontrolled variables (shot placement, size of the individual, etc.).
Every major U.S. police agency uses HP ammunition and, with an odd exception here or there, every expert in the field of terminal ballistics recommends the use of HP ammo in most cartridges. While the military does retain FMJ ammo for the most part (some specialized units do indeed use HP's), this is not because of effectiveness.
The U.S. military continues to use FMJ ammunition in order to comply with the Hauge Convention of 1899 which forbids the use of ammunition designed to easily expand or fragment in a war between signatory nations. Whether or not the U.S. is bound by said treaty is debatable since we were not a signatory nation and we have neither fought a signatory power nor been involved in a formally declared war in over 50 years. That issue is, however, another discussion entirely.
As to your concerns about heavy clothing, it actually enhances the penetration of HP's rather than retard it. You see, a HP bullet that has it's cavity clogged with a non-fluid substance such as clothing, dry wall, or plywood will almost always either expand substantially less or fail to expand all together. One of the biggest problems with older HP bullets is that many of them could not reliably expand if they had to penetrate heavy clothing first. Modern HP bullets expand much more reliably through clothing, but the degree of expansion is often less.
There are only three situations in which I would recommend the use ofFMJ or other non-expanding bullets for self-defense against human aggressors. The first two are rather straightforward and self-explanatory: you cannot readily find good quality expanding ammunition for your gun and/or your gun is not reliable with expanding ammo. Obviously, availability and reliability trump all else so there really isn't much need to expound upon the first two exceptions.
The third, however, is a bit more complex: you are using a handgun chambered for a small-caliber such as .22 Long Rifle, .25 ACP, or .32 ACP. While HP ammo has improved drastically over the last 30 years or so, these small calibers still push the limits of HP technology due to their relatively small diameter (and thus small HP cavity), lightweight bullets, and low velocity. While HP ammo is available for these cartridges, it frequently either fails to expand or, even worse, penetrates shallowly. You can have all the expansion in the world, but it won't do you any good unless the bullet can penetrate deeply enough to reach the vital structures of the target. Because of the shallow penetration so often exibited by small-caliber HP's, I prefer FMJ or other non-expanding bullets for mouse guns in order to ensure adequate penetration.
