you definitely want want with the ability to advance on the tool. As Toolman pointed out, our LA's timing marks only give you 10 degrees and if you want to check total timing in the future, you'll need much more than that. The digital ones are nice as they usually have a built in tach too.
Also, I'll leave it to the 340 experts on the board but you'll probably need more initial timing than what the factory manual calls for due to modern gas blends. My 360 for instance is happiest at around 12 BTDC which is 7 degrees more than what the FSM calls for.