At one point, I used SCO(based on AT&T), and ones like SUN, etc(based on BSD), and DID notice some slight differences in how they work. The IEEE got TIRED of incompatibilities, and came up with Portable Operating System Interface Extensions(POSIX) A LONG time ago. ALSO, BOTH are based on AT&T(BSD licensed AT&T originally). So there are very few meaningful differences inherent in both. Heck, I have an old AT&T unix guide covering everything from lower level libraries, to really odd commands. I used a lot of these on BSD systems with NO problems!
MOST systems these days are based on BSD. I believe SUN, AIX, HP, and Linux are ALL based on BSD. OH, I know! Some purists will say Linux is Linus' own system, with NO unix code, and GNU utilities. The facts? Linus programmed Linux to be close to BSD! Gnu utilities often start from BSD code which comes from BSD. Just look at all the copyright statements.
Steve