I discourage buying the M3i zero, it's just a dodgy flashcart with poor firmware updates.
The firmware looks flashy but it never served me well. If you get one shipped with a broken USB cable (they're very fragile), you cannot flash the M3i, therefore cannot use it!
The Acekard 2i and DSTwo are both great choices.
Acekard 2i is $15, and plays most games just fine with AKAIO firmware. No special features for the most part, just standard things like cheats and soft-reset support.
DSTwo is $35, $20 more expensive but also $20 better. It has fantastic compatibility right now, and boasts a unique integrated CPU that does GBA and SNES emulation.
It features a real-time menu in NDS games, where you can use real-time saves, check guides in .txt or .bmp format, toggle cheats freely or enable slow-motion.
They're both great, it depends what you want your flashcart to do is all.