in terms of features, build and support, the DSTWO is the best flashcart currently in the market. this is wholly irrelevant unless you care about the inner workings of your card, though. emulation is far from perfect, too.
if anything, you might wanna consider battery life- on a OGDS or DSL the DSTWO outperforms pretty much every other flashcart if memory serves, but it's the opposite for the DSi and 3DS: you'll barely manage 2 hours thanks to the CPU constantly hogging the battery.
if you really don't care and just want something that works, you might wanna get a R4i Gold 3DS RTS. you're basically paying for quality when it comes to the DSTWO.