In general, when using SD card for a normal filesystem, the class rating is irrelevant. The random read/write I/O speed is most important.

Sandisk Mobile Ultra (the grey and red card) is tops of most benchmarks I've ever seen for random I/O. Even though they only advertise as class 4 or 6, it is usually more than 10x faster at random I/O than "class 10" cards by other brands (2MB/sec vs 200k/s or less), and can probably be cheaper than those as well. Even the normal Class 2 (or no class stated) Sandisk card is much faster than most Class 10 at random I/O. (If it's genuine sandisk and not a China/eBay fake).

The smaller cards tend to be a little faster, too, for example if you don't need more than 8GB, the 8GB card is probably faster than the 16/32/64GB card (and save money, too).

In my opinion the class rating only matters for devices that write a single continuous stream, like a camera or video recorder.

Also beware that if you partition or reformat your card you could disrupt the alignment from the factory which could have a negative impact on performance. Aligning on erase block size is ideal, 4MB boundaries is usually the safest way (some fdisk tools don't align at all, newer linux fdisk aligns at 1MB which is usually better but not always ideal). Making a backup of the factory partition table before you ever write to the card is even better, so you can learn how they set it up initially and restore that alignment if necessary.

I'm downloading the backupmenu image of PR1.3 right now, as I'm too lazy to rebuild my kernel with MTD support on Anyways, I'll spend the afternoon working on this, first thing to do being to skip mounting /home from the eMMC, instead keeping /home where the rootfs is.