I've built several computers with both. The 840 (non-pro) is actually a little bit faster and boots a second or two faster. Theoretically, the NAND on the m4 should last a bit longer since it's MLC not TLC. m4 has had several issues (5000 hour bug, etc.) but has had quite a few firmware updates and seems to be rock solid stable. I've had nothing but stability with the 840 series.
If you write hundreds of GB to your SSD per day, go with the m4, otherwise, go with whatever is cheaper as recommended.