Figure A is the result of "Envelope Distort" using the "make with warp: arc" option. Especially when working on something using a dramatic warp (like 100% seen here), Illustrator will not render a perfect semi-circle. It isn't obvious in Figure A, but trust me, it isn't perfect. The upside is that it will preserve the spacing between letters from top to bottom. Notice the space between the bottom of the U and the R is good and so is the space at the top of the U and R.
Figure B is the result of typing on a circular path. It keeps each letter from warping and makes a perfect semi-circle. But to get this effect, it rotates each letter slightly. Problems can come up with the spacing between letters, especially at the top. Take note of the U and R!
In many cases, the diferences and problems are barely worth noting... But I suppose knowing the problems in each option, you can choose the one that gives you the results closest to your ideal. I just hope Adobe fixes the warp option (Figure A) so that it makes a better semi-circle.