I like the standby mode more, and I think that is best for being in the pocket. Only keys like the end key will do anything and all it does is give you a message that the phone is in standby mode, and then turns off the display again. With keyboard lock, it's still possible to get random key entries.
+1 for standby mode. Although the recent 4.5 has a bug in which the BB will exit standby if you hold the end key down for a few seconds. It might also work for the other home keys as well.
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I use both because I have my Blackberry password protected. Keypad lock enables my password protection, then I use mute to put it into standby to save battery life and prevent any accidental entries to "make an emergency call" or attempts to unlock it.
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