I'd rather not recommend R4, nowadays, seeing it really isn't worth getting anymore, and I'll explain why, along with recommendations.
Negative:
R4: It used to be the most popular cart when it came out, and it still has some popularity with the consumers. But it's outdated, and all it can do is play games and homebrew, nothing else. It has no SDHC support (Which means micro sds' that are over 2 gigs), and no features. It's not so cheap, compared to the other budget Slot-1s nowadays. You can buy other carts that do more than the R4, at a cheaper price.
M3 real: Not really a good choice. It's incredibly slow, and suffers alot of speed problems. The cart feels like it's made of cheap material, and seems like it'll break under any sort of pressure. The only good side of this, is the upcoming Firmware, the sakura firmware. If you can wait for a long time, since the firmware isn't complete yet, I'll guess it won't be such a bad choice.
Recommendation:
Acekard 2: It's new, but it's a solid cart. It's the degraded version of the Acekard R.P.G, but it's still attractive with it's fast menu, and a great looking GUI. The Acekard team does updates fairly reasonably, and it's a good cart overall.
EDGE: This is also a new card, but it does everything the R4 does, and a little more stuff, at a much cheaper price. It's a little hard to find, but if you can find it, it's worth it.
Supercard DS1 SDHC: The best in features, for it's relatively low price. It has SDHC support, great features like slow-mo, Realtime save, Realtime guide, in-game menu (Same with the Cyclo), and a good team support. It can even make it function to a DSTT.
DSTT: It's cheap, and does the job. It does have a messy menu, because it's not organized at all, but it has the fastest loading time of all. It can load a game around 1-2 seconds. Very fast cart, and seems to have a reasonable amount of updates.
Cyclo: It's the king of the carts, with the best support, that answers your questions directly on the forums, or email, and they're still active to this day. They take the consumer advices, and even tells people what they're going to update next. It also has SDHC support, and has features like Realtime save, and slow-mo. It's highly recommended if you can afford one.
My top recommendation is the Cyclo, but if you can't afford it, then Supercard DS1 SDHC, EDGE, DSTT, or Acekard 2.
All of those are good choices.