Leaving a decock/safety lever in the OFF position makes it a ready-to-use, grab-and-go operation that does not require you to flip the lever OFF. This may be an advantage in higher-stress, self defense situations. It has been debated a lot, and there are endless discussions. This will not be one of them...

Get the Model 59 to own a piece of history. Be aware that it has no hammer block when the decocker/safety lever is OFF.

Get the Model 915 if you are not after collectible/historical significance, if you want to leave the lever OFF, or you want the additional reassurance provided by the hammer block.

If you really can't decide, get BOTH!