Yes, the internal memory is faster. The external Kingston micro-SD cards (in an R4 slot-1 card, for example) usually work fine, but *occasionally* run into problems with demanding ROMs where the game can freeze. A common example given is during the Castlevania DS games, apparently, although I've never tried those ones myself.
My personal experience is that there are some benefits to internal memory -- and never having to worry about speed is one of the best. But there are also some drawbacks to the G6 Real card, most notably less than perfect homebrew support and occasional problems with memory corruption (which I'm sure has something to do with file access when running homebrew, but I can't prove it yet).
Frankly, none of the solutions available right now are "perfect". But I am very happy with my G6 Real and wouldn't trade it for any other card on the market... at least, not right now.
If you are looking for a solid card, this one or the R4 are two of the best choices in my opinion.