As a DS-X owner, I must say that I am totally jealous of the R4 I got my girlfriend for christmas.
DS-X has never given me any problems, and I don't think I would sell mine to get an R4, but its just the little things that make it look awesome.
For instance, The R4 Starts up immediately after the initial Nintendo DS intro screen, bypassing the DS main menu. and the menus within the R4 are really fast. Whenever we turn our systems on at the same time, hers always gets to the game faster. Plus the DS-X is really slow while navigating through its menus, and has to re-search and load up all the games you have on it every time you start it up.
Which really doesn't matter, i mean its kind of annoying, but I guess thats just it, it doesnt matter. Pick whichever, but the R4/M3 are way cheaper than DS-X.
Oh and you can get back to R4 Menu without having to restart your DS which is really awesome.
So to answer your question, I would say R4/M3 over DS-X