I would like to make a SIO2SD - SDrive comparison.
INSTALLATION: both are easy to install, you only have to put a loader/create a folder on the SD card; then you just copy all your .xex and .atr from your PC without compatibility problems.
PROGRAMS SELECTION: both offer on screen selection, SIO2SD let you select programs from the lcd display too (useful to swap disks or create them on the fly).
ON SCREEN MENU: SIO2SD displays long filenames of all files, SDrive displays long filenames of selected file only.
NUMBER OF DRIVES/DISKS SUPPORTED: SIO2SD 15 - SDrive 4 (f.e. Alternate Reality, The Dark Crystal, F.R.E.E., Gunslinger need more than 4 disks)
SPEED: according to this thread, SIO2SD seems faster but IMO more tests need to be done.
SD CARDs: SIO2SD up to 32GB - SDrive up to 2GB
LAST SOFTWARE UPDATE: SIO2SD 2012 - SDrive 2009
PRICE: SIO2SD parts are more expensive but SIO2SD devices strangely seem cheaper. EDIT: recently AtariAge user santosp started selling cheap SDrives (look my topic for newbies).
CONCLUSIONS: SIO2SD and SDrive are both good devices. SIO2SD has the advantage of drives/disks supported (15 instead of 4, important for some games contained in more than 4 disk), disks selection/creation on the fly thanks to lcd display and loader long files names display (all files, not selected ones only).
Please feel free to fill voids, add fields and correct me if I am wrong.
I will update this post accordingly.