Samsung - 840 Pro 256GB vs Samsung 840 EVO 250GB 

It really depends on your workload and needs. The pro has higher random access speed along with better 4k speeds along with a higher endurance rating. The Pro would be better for high usages with lots of files such as 3d modeling, web content creation, audio and video editing. You would only notice the differences over a large period of time or large workload. Gaming and every day use, you wouldn't notice a thing.