Maybe the place to start is asking what is it you want to know, and that is different for different users. A mechanic troubleshooting a car may just want to exclude the battery or alternator as causes or not, most consumers want to know if the battery is still OK and maybe a guess at how much its used up or how long it may stay good.
Another question is whether or not you need to be able to test without a car. If you are only testing in car, then a fairly simple voltage monitor while starting the car may tell you as much as a pile tester, but maybe less accurate.
If you want something small and portable, that means conductance testing.
Smart alternator checkers look at waveforms to detect single diode failures etc., and most of the battery checkers seem to be limited to simple voltage level checking, which is plenty good to detect most failing alternators.
Add in a shop situation you want the test to happen both accurately and quickly, and since you do it a lot a large premium in price for a small set of features, like printing, may be OK.
