Explanation in order
keep hearing from the fans of the 9mm that there are much more improved rounds on the market for sd purposes.
Might I inquire as to where you keep hearing this from? Or if you heard it from a particular place?
In any event, usually when someone says something along the lines of "a 9mm is perfectly fine for self-defense purposes with modern well-designed ammunition" they are referencing ammunition produced post 1986 with an eye towards meeting the new standards established by the FBI, and later organizations.
1986 saw an incident with the FBI where officers died in a firefight with some well armed and tactically proficient bank-robbers. The FBI blamed the death of the officers on the failure of ammunition design. Prior to 1986 the FBI chose ammunition based on the RII (Relative Incapacitation Index) which favored expanding ammunition, but took no account for bullet penetration, or for reliability of expansion. The FBI testing protocol and IWBA testing protocol test for all of these factors. The Hydrashock line of bullets tested very well in initial testing, and consequently sold like hot-cakes for years (and still do), despite the fact that other bullet designs far eclipse the outdated hydrashock. The process of development has been evolutionary, every few years a new design of the same bullet, or a new bullet design entirely gets just a little bit better. It's been over 20 yrs since 1986, and this tiny incremental changes add up to quite a bit.
The upshot of all of this is that bullets that meet IWBA and FBI test protocol expand *much* more reliably than older bullet designs (because they were designed to do so), and are actually designed to penetrate a set minimum distance, in addition to expanding. A well designed 9mm bullet today is much better than .45 FMJ, older JHPs, and newer JHPs that are poorly designed.
The downside is that not all bullets are designed to perform to IWBA or FBI standards, and there's no easy to find label that will indicate if a cartridge meets that standard, you have to find evidence someone has conducted the testing yourself.
If you're a police officer, you can ask the FBI, us civvies are kinda on our own. Lucky for you, I've got a line on a link:
http://www.tacticalforums.com/cgi-bin/tacticalubb/ultimatebb.cgi?ubb=get_topic;f=78;t=000964
Presto.
As others have suggested, before deciding on a load it may be a pretty swell idea to buy a bunch of that particular sort of load, to be sure your weapon digests the ammo without a hiccup. FMJ that feeds beats the heck out of "thor's hammer" that jams. More important of course is how well you can place shots on target under stress.
-Morgan