Handlebar stem : height vs length
The answer is trigonometry. Most bikes have a headset angle of about 74°, give or take a degree. Over the adjustment range you're looking at that degree doesn't make a difference. 

Here, tan(74 degrees) = height change / reach change = ~3.5. So to get a centimetre of reach change to need to lift the stem 3.5cm, putting the stem up reducing the reach. Or you can invert that and say each centimetre you lift the stem changes the reach by about 3mm.

Roughly, the stem that's about 1cm longer but 3cm higher will have the same reach. But 3cm higher will feel quite different, and because of the way your body curves to reach the bars it will probably feel as though the reach is shorter.