Oooops you forgot the options!
But I can still give you some ideas:
1. Columbus was Italian and represented (and was paid by) Spain, while Dias was Portuguese and served the Portuguese King
2. While they both were interested in finding the way to India, Dias decided to go around Africa (East) and Columbus wanted to go towards West - hopefully around the Earth
3. Dias discovered the Southern Tip of Africa, while Columbus discovered a new continent
