I thought SC1's success was that there were 3 distinct races for its time, whereas most of the other RTS games including Warcraft II were pretty much a game with very similar races with very little difference between them.
Personally i like the first one alot better for numerous reasons after playing the beta, but everyone is going to play the second. So if you like RTS that is quick and offensive in general, i recommend this. However if you like more prolonged battles and a lil more terrain/squad usage, i recommend Company of Heroes or Warhammer40K instead.