If you want to use homebrew, get the M3. If you just want to make backups of your carts and play them from a memory card, get the DS-Xtreme.
I bought a Passcard 3 with my M3 as a bundle from mrmodchips, which gets you booted to the M3. From there you can run FlashMe which lets you flash the DS firmware. Once that's done you no longer need the Passcard as the DS will boot directly to the M3 if it's there.
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moto