I have R4 and SC DS, and i only use the R4. I let my little brother borrow the SC DS.
True, SC DS has more features, but many roms just don't work with it when you trim them, and ALL my roms have worked on R4 when trimmed.
I like that the SC DS OS is based on moonshell, since then you can play dpg's without having to boot into moonshell. However, the trimming thing has annoyed me greatly, because then i couldn't fit as many roms on each MicroSD.
If i were you, I'd go for R4. Even though i have some weird problems with it that no one else with R4 has (DSOrganize crashes every time i try to go into the file browser, and tells me "Fatal Error! DSOrganize has run out of memory!" on the top screen after a few seconds.) I never had those problems with SC DS.
I haven't used my SC DS since before they added AR cheat support, so maybe they fixed the issue with trimmed roms. I don't know. I doubt it though.
Slow-motion sounds nice, but not really useful unless you want to make a game easier (which I'm against since it counts as cheating).
Fast-forward however is something they should have, to skip long and boring dialogs in RPG games and etc., too bad the DS CPU's can't handle it.