By looking at the numbers
. I don't have an x51.
In the Axim, according to Spb Benchmark's Speed Index, the RiData 2GB 150x and Transcend 4GB 150x are fastest closely followed by the SanDisk Extreme III and Ultra II. The SDs are faster than the CFs. At the low end the MicroDrive performs very well compared to the other CF cards. I cannot say that I have felt real differences when using these cards in the PDA because I have not devised a systematic test for that.
Anyone reading this who has an idea how to test real world card speeds in a PPC in a reproducable way, speak up.
In a card reader, according to HDBench as well as real life tests, the Extreme III cards - both SD and CF - rule, followed by the RiData. The Transcend 150x is a distant third, not much better than the 70x and 60x cards. Forget about the MicroDrive.
Of the cards I have tested the RiData has good or best read and write speeds both in the Axe and a card reader, and costs less than the Extreme IIIs.