At the IDE transfer rates of an A600 or even the A1200 I don't think the "speed advantage" of a CF card matters at all.
Should you ever want to install a Vampire 2 or a Furia accelerator in your A600 the SD card adapter doesn't get in the way.
In case of an Amiga it is not about SPEED but also about ACCESS time, HDD will have the same "raw" speed, but when I switched to CD/SSD solution my Amiga boots INSTANTLY.

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For an Amiga software HDD speed is not that important. Why? Because most of Amiga software is relatively small. Speed will make a difference when you want to transfer 1000Mb of mods to your Amiga... then 1Mb/s... IS slow.