With the DSTWO+ you basically get two solid flashcarts for the price of one, and in addition to that it's cheaper than the Gateway. You can use one cart for all your DS, 3DS, SNES and GBA needs (though the latter two might still be better in native 3DS mode on a n3DS) and never need to swap it out.
It IS a clone though, it's not endorsed by Gateway, and they might stop it from working with future Gateway updates. That's the risk you're taking when buying a DSTWO+.
However, SC have a great track record, and I don't think they would risk their reputation on a clone unless they were certain that it can't be blocked by a Gateway update.
Gateway has made some rather questionable decisions in the past, not to say that SuperCard cloning another company's cart isn't questionable, but I trust SC team over Gateway.
But, bottom line is, the DSTWO+ is relying on Gateway to provide future updates with new features, support etc. and if everyone buys the DSTWO+ instead then Gateway have no incentive to continue improving their cart, so by buying a DSTWO+ you are not just hurting the Gateway team and the chances of them providing updates and improvements, but you are hurting your chances of getting further updates and improvements for the DSTWO+ as well. If you have the money you should support the Gateway team who did all the work that made this possible.
Either that, or get CFW. If you are on the lookout for a DS flashcart though, getting a DSTWO+ over a DSTWO is a reasonable decision since you get more features and a faster onboard CPU for not much more money.