Go with the reach. Stay with a small or a 15 or a 16 or whatever it is for you. MTB's do have more standover, it's true. That's still convenient with a commuter bike, it gives you something like stepthrough when you have panniers or a kid seat.
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Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."