A straight six engine is one where all six cylinders and in one
straight line (hence the name straight six) whereas a V6 is built
in a V-shape with 3 pairs of cylinders.
A straight six engine tends to be smoother than a V6 engine,
because the straight six does not require balance shafts like the
V6. However, the straight six is obviously longer and therefore
more difficult of an engine to place in cars than a compact V6
would be.