Two very different races. Here's every stat, side by side, to help you pick.
Indicative — based on GPX data. Both courses normalised to start at 0m.
✓ marks the better number on that stat.
These two could not be more different. Berlin is a dead-flat, record-built loop that's produced more men's world records than any course on earth. Boston is a point-to-point run from Hopkinton into the city with 265m of climbing and a net drop of 135m — famous for the Newton hills and Heartbreak Hill in the second half, and for being the world's oldest annual marathon, run continuously since 1897.
Entry is another split: Berlin is a straightforward ballot, while Boston is the one Major you have to qualify for, by time standard, making it as much an achievement to enter as to finish. If you want a fast, controllable PB attempt, Berlin is the easier calculation. If you want the marathon with the deepest history in the sport and don't mind the hills, Boston is the one that means something different entirely.