Well, the Gateway is the only 3D mode flashkart i own, so I can only relay information in regard to the others I've read from others:
As of now the r4i gold deluxe has the same features and limitations as the Gateway (afore mentioned save bug, some games (Pokemon and Animal Crossing) don't work because they have a different way of saving, only one game per microSD card).
Some people (me included) speculate that the clone cards will not be able to offer the features Gateway has announced: multirom support (multiple games per microSD card) and saving for Pokemon and Animal Crossing.
Therefore my advice: If you don't want/need a 3DS mode card now, wait another one to two weeks, by then it should be clear.
If you want/need one now: If you don't care about having multiple games on one microSD and/or don't care if you can play Pokemon, Animal Crossing and possible future games with their save system or like to gamble, buy an r4i gold deluxe or 3dlink.
If you want a high probability (no guarantee) to get multirom and newsavetype game compatibility buy a Gateway