If you use a decent SD card not some prehistoric 2MB one the runtime experience will improve. There is also lots of scope to increase the SD card device speed via device tree if faster speeds prove reliable. That's always a gamble with Android boxes, which is why the card device in the currently experimental device-tree is using slower-but-safe defaults.
NB: I'd rather have dental work done without anaesthetic than engage in supporting emmc installs on Android boxes that we have no boot sources for. Under the legacy kernel it's a major pain in the arse to support but technically do-able. On an upstream kernel where the kernel cannot understand the partition scheme or see any of the partitions on emmc it's an entirely different game.