Primary = green and purple spots
Secondary = blue and yellow spots
Primary is typically in used condition so if you like the primary colors and you actually find them in mint condition, then pick them up fast. Secondary is typically in much better condition because they weren't put on the casino floor yet and is much easier to find in mint condition.
In the case for these particular chips though, both primary and secondaries are fetching about the same price in mint condition. The mint primaries might get you $20 more per rack.