Nds-card.com is a good shop, but the free shipping can take 2 weeks. (from China to US)
That card (r4i-sdhc.com) is decent, the main problem is the menu is a little sluggish and it can't see .nds games more than one folder deep. It doesn't let you browse folders, it'll just list the roms/programs that it finds. Depending on how organizational you are you may not like these points. But what it does have is RTS (save-states) with multiple slots. Compatibility is not perfect either but it's probably second-best to the DSTWO's savestate compatibility. In either case savestates may not work with demanding games.
You may not want to buy the SD card bundled with the flashcart because you can get them cheaper seperately. It's up to you though, if you want the convenience of buying them together. Be wary of fakes though.
Other cards:
R4i Gold 3DS (r4ids.cn)
http://nds-card.com/ProShow.asp?ProID=149 - This one is recommended the most by users here because it has WOOD support, which features a fast, clean menu and many features such as multiple save slots and game language select. Weakest point may be the RTS, only one slot and I haven't heard it being very good.
2016 R4i SDHC Dual-Core (r4isdhc.com.cn)
http://nds-card.com/ProShow.asp?ProID=507
This card is
very underrated (in my opinion). It supports WOOD just like the above card and can be had for cheaper. The
only difference is lack of RTS, which you might not want anyway.
(If you would like to buy this from me please pm )
Supercard DSTWO:
Built in processor lets you emulate GBA and SNES pretty well. Probably the best savestate support. Menu is pretty good too. Has multiple save slots, save state slots, ingame-cheat menu, slow motion, perhaps more I can't think of right now. Downsides: the premium you have to pay for it, and increased battery drain.
EDIT: Also for this card:
http://nds-card.com/ProShow.asp?ProID=291 I would like to know if it will work with all the Ace Attorney games released for the DS, Mario Kart, Mario Party,etc. Thanks!
At this point game compatibility is near-perfect for all cards, it's about what features you want really.