To do this I needed write and read benchmarks. So I used the ones that MrRob developed in another thread.
Cards used in testing:
133X is
http://www.bhphotovideo.com/c/product/756124-REG/Lexar_LSD128CRBNA133_128GB_Professional_SDXC_Card.html
400x is
http://www.bhphotovideo.com/c/product/841484-REG/Lexar_LSD128CTBNA400_128GB_SDXC_Memory_Card.html
Price difference is $12. I needed another 128GB card for a camera and thought I'd benchmark it. The UHS-I card isn't staying in my Pandora.
Each card is formatted with a single 120GB ext2 partition. My Pandora is clocked at 800Mhz.
Write benchmark: dd if=/dev/zero of=/media/sdcardname/file.img bs=1024k count=500
Card, slot, iteration, result.
Lexar 133X 128GB SDXC card, left slot, i1, 524288000 bytes (524 MB) copied, 57.9436 s, 9.0 MB/s
Lexar 133X 128GB SDXC card, left slot, i2, 524288000 bytes (524 MB) copied, 56.0055 s, 9.4 MB/s
Lexar 133X 128GB SDXC card, left slot, i3, 524288000 bytes (524 MB) copied, 55.5187 s, 9.4 MB/s
Lexar 400X 128GB UHS-I card, right slot, i1, 524288000 bytes (524 MB) copied, 57.7025 s, 9.1 MB/s
Lexar 400X 128GB UHS-I card, right slot, i2, 524288000 bytes (524 MB) copied, 56.4106 s, 9.3 MB/s
Lexar 400X 128GB UHS-I card, right slot, i3, 524288000 bytes (524 MB) copied, 60.5005 s, 8.7 MB/s
Lexar 400X 128GB UHS-I card, right slot, i4, 524288000 bytes (524 MB) copied, 61.3672 s, 8.5 MB/s
Lexar 400X 128GB UHS-I card, right slot, i3, 524288000 bytes (524 MB) copied, 60.4797 s, 8.7 MB/s
An odd thing happened during the write benchmarks - the UHS-I card appeared to be steadily getting slower. So I kept benching it to see where it would bottom out. It does have more variability in write speed than the regular SDXC card.
Read benchmark: dd if=/media/sdcardname/file.img of=/dev/null
Card, slot, iteration, result.
Lexar 133X 128GB SDXC card, left slot, i1, 524288000 bytes (524 MB) copied, 30.501 s, 17.2 MB/s
Lexar 133X 128GB SDXC card, left slot, i2, 524288000 bytes (524 MB) copied, 30.4598 s, 17.2 MB/s
Lexar 133X 128GB SDXC card, left slot, i3, 524288000 bytes (524 MB) copied, 30.5322 s, 17.2 MB/s
Lexar 400X 128GB UHS-I card, right slot, i1, 524288000 bytes (524 MB) copied, 30.2341 s, 17.3 MB/s
Lexar 400X 128GB UHS-I card, right slot, i2, 524288000 bytes (524 MB) copied, 30.2754 s, 17.3 MB/s
Lexar 400X 128GB UHS-I card, right slot, i3, 524288000 bytes (524 MB) copied, 30.2666 s, 17.3 MB/s
Findings:
Both of these cards are fast and have 128GB capacities, but from the perspective of the Pandora - there isn't any real difference in performance. If you're planning to use a card exclusively in your Pandora, there isn't anything to gain by spending the extra $12.
Grench