The difference between a
simple emission checker (like a Heath TC-1, 2 & others) and a mutual
conductance* checker is very important. An emission checker tests for
shorts & cathode (or filament) emission only. The meter reading gives a good-bad
reading by virtually tying all the elements of a vacuum tube
together (except the cathode & heater) and testing the tube for
cathode emission by subjecting it to test as a diode (or
emission checker will basically tell you whether a tube is
usable or not and give you an indication how strong is the
cathode emission. Both types will do a good job testing for
shorts and leakage.
This is not to say that an emission tester is of no value, even the best tube testers can mislead and test a tube good even though it may not function well in some circuits.
* Gm.....the symbol for mutual
Summing up, if you want a tester for occasional use for testing the tubes in your own equipment, you may no want to invest in the more expensive mutual conductance testers. On the other hand, if you have lots of tubes and want to sell them on eBay, the mutual conductance value (as read on a properly calibrated mutual conductance tester) supplied along with the tubes you are offering is a must.