NAND is arguably the most common for industrial application, however while NAND Flash has been a popular storage technology for many years, it’s become more complex to design into an embedded system as it reaches higher densities. That’s where eMMC Flash—which has its own integrated NAND controller—can come in handy. My suggestion is eMMC. please also the boot sequence for i.MX6UL and eMMC, https://community.nxp.com/message/892367
