Obviously you are not talking about SBCs but digital cameras instead?
For a camera sequential write speeds are important. For a linux system utilising an SD card or eMMC NAND for the rootfs or even worse a /tmp or /var/log directory this isn't that important.
The Guitar seems to be limited regarding sequential write/read speeds to/from SD cards in the usual 20 MB/s range (unless you permanently try to write large amounts of data this doesn't matter that much). The eMMC shares the sequential write limitation and is 2.5-3 times faster when it's about sequential reads. Does this matter for an SBC? Still not that much.
What's far more interesting is how the SD card or eMMC NAND handles short writes and random I/O. And there you can not compare between the Guitar's eMMC and 'SD cards' in general. Because the latter perform totally different and this has nothing to do with the 'Class'. You can get 'Class 10' cards that suck regarding IOPS compared to 'Class 4' cards that might suck regarding irrelevant sequential write speeds.
As a rule of thumb: eMMC is faster than most SD cards. And that's it.
Way more important if your device's rootfs is lying on SD card or eMMC is the way it will be accessed. And the best way is to avoid access where possible and use filesystems that care of the characteristics of flash based media. A good attempt is to use F2FS for flash based media (do a google search for 'f2fs vs ext4' to get an idea) but can we use it? No, LeMaker's kernel has no support for it. The other problem is: Since we're stuck with an outdated kernel 3.10.37 on the Guitar you won't get mature driver code (F2FS was introduced in kernel 3.8 and using filesystems you don't want to use fresh code but mature instead. And you want all the fixes contributed to the kernel in the meantime -- it's 2015 and we're on version 4.2!).
Since we have to deal with ext4 at the moment it would be a good idea to use a high commit interval, to use tmpfs and ramlog where possible and use real storage (in the Guitar's case connected using USB 3.0 if that will be ever possible) if your device should act as a file server.