Like Smack said, you rly need a good player on behe if you want him in lineup, a bad player will more often fail fissures,which can be fatal.
Magmus is more likely for the not so good players, i prefer playing magmus over behe. like gad said he is easier to land stuns with and got the steambath.
the ultis:
without pk behe got far the best ulti, because you cant interrupt him.
im playin in the lower-mid bracket, where alot of magmuses fucks the ulti up.
all in all i would say they are situational.

this is just my opinion, remember im low-mid skilled :))