Fixed delay or fixed rate?

A fixed delay timer script (the default) waits for the given delay between each script invocation. This means that the script's rate will actually be the delay plus the amount of time it takes to execute the script. This is the safest option since it prevents a script from mistakenly running continuously because it takes longer to execute the script than the delay.

Fixed rate scripts attempt to run the script at a fixed rate relative to the first execution. If the script takes too long, or there is too much background process, this may not be possible. See the documentation for java.util.Timer.scheduleAtFixedRate() for more details.

Shared thread or dedicated thread?

All timer scripts for a given project that choose Run in shared thread will all execute in the same thread. This is usually desirable, to prevent creating lots of unnecessary threads. However, if your script takes a long time to run, it will block other timer tasks on the shared thread. The rule of thumb here is that quick-running tasks should run in the shared thread, and long-running tasks should get their own thread.

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