Screening compounds are generally used for screening purposes, while building blocks are typically serve as starting points for further synthesis steps. Guaranteed purity is typically lower for screening compounds and they are usually also available in smaller quantities.
Which one you should buy depends on what you are planning to use the compound. For general screening (to assess if it is active on a specific target enzyme/receptor) or if smaller quantities and lower purity is acceptable for you I reckon to buy it as a screening compound.
Robert Kiss
CSO
mcule.com