Burn rate!
There's "fast" powders, and "slow" powders. Here's the list:
http://www.imrpowder.com/PDF/Burn Rates - 2015-2016.pdf
For a given cartridge with a given bullet weight and a given seating depth, you end up with a given capacity for powder. Generally speaking, the more powder capacity the slower the powder that given combination will accommodate. You can use the faster ones also, but you'll be capped as you go up the burn rate chart.
Fast powders reach maximum chamber pressure with lower charge weights than slow powders. Usually the maximum velocity is reached with the slower powders listed. So in the .357 for example, if you load to maximum listed, W296 will have a higher velocity than Unique, which will have a higher velocity than Bullseye. None of this matters if maximum velocity is not your goal.
Accuracy is another story, and for the most part accuracy is not dependent on the burn rate of the powder. Slow can be just as accurate as fast. Fast has the benefit of using less powder. It also has the potential to be a disaster if you double charge a case. But this is somewhat of less concern if you're loading .38 spl and shooting it in a .357 gun. You have sort of built-in over-pressure protection since the .357 is rated for a much higher pressure. When looking at .38 loads I often check the maximum .357 load with the same powder. Often a double charge of the .38 load with a fast-ish powder is pretty close to a maximum .357 load (though maybe a tad over!). Not something you want to do, but I like to know anyway. It would probably not blow the gun, though you might jug a chamber. The smaller case capcity in the .38 vs the .357 will raise pressures some too. With the slow powders it's usually impossible to double charge a case as they tend to fill it pretty full.
As an example I've loaded .38/.357 with fast powders, and slow powders. The super fast powders may only use something from less than 3 gr. to maybe a little more than 4 gr. Slow powders get up into the teens.
Somewhere in the middle for a given cartridge is a good place to start. I tend to throw out the fastest and the slowest powders listed. In this case if we're talking about .38 spl target/general purpose loads there's lots of known results from 100 years of reloading it by umpteen thousands of shooters. I've loaded everything from Tightwad to Universal in the .38 Spl. My favorite is W231, but I've had equally good accuracy with Bullseye and 700X. Look at your load information, look at the burn rate chart, and pick one. If you can't find a load that works for you try another powder. But I can tell you, one of the great things about the .38 spl is just about anything listed will work well, and the best will work very, very, well.
Powders have other characteristics that reloaders like or dislike. Like how accurately it meters through their powder measure (for me W231 good! 700X horrible!). There's also some new powders out that are completely different in other ways. I've read that CFE Pistol is great for the .38 Special. It's kind of higher on the burn rate chart than I usually go so I don't know. The there's Trailboss, which is really different. It's fairly fast on the burn rate chart but a very bulky powder. I have only used it in .41 Magnum for low velocity loads. The 6.0 gr charge mostly takes up all the available space. Shoots great, but this powder is different.
