Naturally we have "fanboys" (if you don't like the expression, ignore it, just a way of characterizing supporters of a determined flash cart) of all popular carts. No one in their right mind would support the DS-X, seeing as it's updates are throughly bad and take a long time to be released.
The R4 has a lot of supporters, me being one of them (SUPPORTER, not fanboy), and I did have a DS-X, which I liked to use until a few months ago when 1.1.2 was released. Completely screwed up my Pokemon game, which I DID NOT LIKE A BIT, despite having an older backup which I had to resort to. So I got an R4. I was thinking, hey, it seems a little better than the DS-X, soft reset, AR codes, they're all cool funcions, but you can go pretty well without them, right?
So I got my R4 a few weeks ago. I was amazed at the easiness of just updating the firmware and setting things up! No buggy FPGA, EEPROM, updating issues, etc. Just copy to MicroSD and load. Very easy.
Game loading times have made my friends awe in surprise because I had shown them my DS-X a few months ago and it was slow (still fast, though, but R4 beats it).
Soft reset just made my friends go O.o, and I like the feature a LOT. It's extremely handy when I just want to play another game without the hassle of hard-resetting my DS (doesn't take long, but hey, if time can be saved, why not? ^^).
Action Replay codes are a good feature, surely, but I don't use it a lot. Why? Because I like the pleasure of beating the game, instead of skimming through it in 2 hours (e.g. main story, mode, etc. complete) with some superpower 999HP or all items in a jiffy). I use it sometimes, but only when I'm really annoyed because I can't pass a certain section of the game.
So my point is practically go for the flash cart that most suits you. I went for the R4 - excellent features and price. And there are legions of fans for the R4. But despite this, take a look at other flashcarts. Because the scene is not only centered around the R4. There are a lot of other (really good) flash carts out there like the Cyclo Evo, EZ V, SC-ONE, G6DSR, etc...it just takes some research.