Depends on the version of the automatics. We have very different 300s in our fleets and some convert from VS to LVL CHNG when the speed differs by only 5 kts from the selected speed, others quite happily fly into the shaker or clacker. So dont trust it, you have to be aware what the plane's doing and keep yourself in the loop.
In general V/S is more controlled in terms of at what place do i want to be at what level, but can be more uncontrolled in terms of speed. During climbout i usually use LVL CHNG except for small altitude changes or the last 1000 ft, during descent i usually start out using VS and revert to LVL CHNG when the speed becomes controlling, for example given speeds in a terminal area while still keeping an aye on actual VS of course since we have SOP limits about VS in certain altitude differences above the MSA.