I own both of the carts you're discussing. The EZFlash spends more time in my DSi than my SCDS2 does, it just gets me to my games faster as there is less crap in the way. I also prefer the interface: I use the small icon mode, something the SCDS2 does not offer. On an 8GB SD I want to see more than 4 games at a time. The EZ5 also starts up w/ the last game you played already selected, the SCDS2 does not. The EZ5 interface is also faster... But don't think I don't like the SCDS2.
The RTS is far more stable on the SCDS2 than it is on the EZ5. If that's the most important feature to you, the SCDS2 is the way to go.
The cheat searcher is great, but w/ the unholy number of cheats
that can be found on the temp cheat forum I rarely find the need to create codes myself. It IS almost always the reason that I switch from the EZ5 to the SCDS2 though when I feel the need to do so.
Slow motion is something the EZ5 lacks, but I've yet to use it w/ the SCDS2 beyond testing to see how well it works; cool feature for those that care to use it, I'm not one of them.
Both have in game guide features and I really don't prefer one to the other. I actually tend to use my PSP for this becuase of the higher resolution and thus characters per line. I don't have to fight w/ tables and such as much to get them to display reasonably on the PSP as I do on the DS.
For GBA, most well recognized games I've tried work pretty well via the SCDS2's emulation, but just about everything runs better on my PSP, so I tend to use my PSP for GBA emulation on the games that run well via emulation. More obscure games that don't have speed up patches and really hammer the hardware (Payback for example) run like crap on either system so I fall back to my DS lite and a GBA flash cart for those. SNES games are far better emulated on the PSP so I'm not using the SNES emulator on the SCDS2 at all. If you don't have a more powerful portable than the DS and/or a non "i" DS than the SCDS's emulation should be a pretty compelling and not overlooked. It's not that it's bad, it's just that I have better solutions than the SCDS2's offering.