With the release of Palm operating system version 4 and the m500 series and m125 PDAs, Palm has given users the ability to add expansion cards using the built-in Secure Digital/MultiMediaCard (SD/MMC) slot.

Since you can use either an SD or MMC on your Palm, users who're looking for more storage space will have to decide if they want to get one or the other. Which one should you get and what are the differences between the two?

To help you decide which format is suitable for you, we've examined the similarities and differences between both formats.

Size
Both MMC and SD cards are flash memory cards that're used with digital devices. Essentially, they work like floppy disks where you can save, store and transfer files to another device simply by inserting your card into the SD/MMC slot.

Weighing only 1.5g, these postage stamp-sized cards measure 32 x 24mm. However, the SD card is thicker at 2.1 mm compared to MMC's 1.4mm.

Being 0.6mm thicker, the SD card is probably more durable. However, the difference is so slight that when it comes to day-to-day use, the slight difference in thickness probably won't matter.

Speed
The biggest difference between an SD and MMC is the transfer rate. While an MMC can transfer data at a maximum rate of 2.5MB per second, an SD card can transfer four times as fast, at 10MB per second. Do note that these are the maximum transfer rates. Whether the card can reach that speed depends on the device using the card.

If you use a card reader to transfer files from a card to your PC, the difference in transfer speed is noticeable. But when using the cards with your Palm, the transfer speed won't make much of a difference since the Palm does not reach this speed during normal usage. However, we noticed that if you copy large files (about 4MB) to and from your handheld to the card, the transfer seems to be slightly faster with SD cards. Since you probably won't be transferring very large files in your day-to-day operation, the difference in speed is something you can live with.

Security
Since SD stands for "Secure Digital", it doesn't take much for one to know that another major difference between an SD card and MMC is security. While the MMC has no security features, SD cards have a built-in security element that allows cryptographic security for copyrighted data. This means you won't be able to copy information or applications that are pre-installed on a card and pass this on to other users. This feature is most useful to companies which decide to distribute data on SD cards as this will prevent someone from copying applications or information and using it on another card or device.

The security level of the SD card also complies with requirements set by the Secure Digital Music Initiative (SDMI), a forum that brings together folks in the recording, consumer electronics, and information technology industries to develop technology specifications for protected digital music distribution. Hence, music files can be distributed without the worry of being copied to another device.

The SD card also has a write protect switch on the case. Similar to your floppy disks, you can manually protect your files from being modified or overwritten by flipping the catch found on the card.

The security features on the SD card is actually most useful for those who intend to sell cards with content since your data and software cannot be copied illegally. But for a user who wants to use the SD card for storage or even run apps off the card, the additional security features are not really useful. Also, the copyright protection feature takes up about 100K on your card, meaning that you've less storage space to work with.

Similarly, the write protect feature seems to be quite redundant since you'll need to read and write to the card frequently anyway. So unless you absolutely want the security features, the MMC is sufficient.

Price
Since the SD card has more features than a MMC card, it's no surprise that SD cards will cost more. When we check with local retailers, we found that the price of a 64MB MMC card cost around S$130 while a SD card with the same capacity cost S$170.

However, a check on Flashmemory.com.au, an online retailer of flash memory-related devices, reveals that the price difference is not as great. A 32MB SD card goes for about US$45 while the MMC version goes for US$36. For the 64MB cards, both the SD and MMC version retails for around US$76.

The bottom line
Although a SD has more security features and is faster, it seems that under normal circumstances, an MC should be sufficient for most of your day-to-day Palm usage. Unless you want to use the card with devices other than your Palm, such as a SD/MMC card reader connected to your PC or a digital camera, then the faster transfer speed of the SD card is more desirable.

Ultimately, it boils down to price. If you can get an SD card or MMC with the same capacity at the same price, by all means, go with the SD card since it will give you more options. But if the price difference of an SD and MMC is more than US$10, we suggest you stick with the latter.