QUESTION:
My question is why does it take longer for the MGS to get to Mars than the
Rover when the MGS lanched first?

ANSWER from Pieter Kallemeyn on February 17, 1997:
MGS is travelling on a different trajectory than Mars Pathfinder.  The MGS
trajectory goes further around the Sun, therefore it takes a little longer to
arrive at Mars.  The reason MGS does this is so it arrives at Mars going slower
than the arrival speed of Mars Pathfinder.  This means MGS will save fuel when
it goes to use it's thrusters to slow itself into Mars orbit.

Since Mars Pathfinder doesn't go into orbit, it doesn't have any restriction on
arrival speed, so it can travel the faster, more direct route to Mars.

- Pieter Kallemeyn