In general, you'll find the +30 deg stem provides a more upright, comfort-oriented fit, whereas the +/-6 deg stem puts you in a more performance-oriented riding position (standard for a typical road bike).

The length of the stem is measured along the stem tube. So, if you compare a 100 mm length stem in +6, and +30 here is how the Reach (X) and the Rise (Y) compare:

You can see that the +30 would put the handlebar 40 mm higher and about 13 mm closer to the rider than the +6. But, the stem is not alone sitting on the ground, it is on the bike attached to the steerer tube so the above triangles need to be rotated into the frame of reference of the steerer. A typical steerer is ~16 deg tilted back which would increase the effective angle of the stem by that amount. If you compute the X and Y values with this additional angle you see that the +30 would put the handlebar about 35 mm higher and 23 mm closer to the rider than the +6.