First lets see the price comparison between both of these:
Kingston 16 Gb class 10 microSD card - $AUD 9 - $0.56/Gb
Hardkernel 32 Gb eMMC module - $AUD 61 (16 Gb is $AUD 47) - $1.90/Gb ($2.93/Gb for 16Gb)
So the eMMC module is about 3.4 times more expensive. That's a fair price difference!
So lets see how fast the XU4 booted up in each case (running the default Linux install (ubuntu-15.10-mate-odroid). Time was measured from when the device powered on to when the login screen appeared.
Kingston 16 Gb class 10 microSD card - 41.67 seconds
Hardkernel 32 Gb eMMC module - 25.5 seconds
So the SD card takes roughly 1.6 times longer to boot.
So price wise the eMMC module is completely not worth it, at least if you're looking at boot time performance. On the other hand, I do not want to be waiting an additional 16 seconds for the device to boot (my Mac boots in 10s), so getting the eMMC module is worth it in terms of reducing frustration levels! I will be doing further tests between these in the future so keep an eye out for more articles.