MIDI is the connection between MIDI instruments and your interface. You connect for example your keyboard to the interface and the signal is sent into your pc through USB. If you only have usb midi instruments, you do not need midi i/o
For the interface just think about how many inputs you will require.
Also, I am using a cheap little mixer myself (Behringer Xenyx 802) and I realized I never use the panning or EQ options, I do all that on my computer which is why I am planning on buying an interface now. So if you don't think you'll use these options, buy an interface.
If you are sure you will only use the inputs and outputs and don't really require extra feutures, then buy an audio interface, it is much more compact.
From what I have heard, the Focusrite Scarlett series are great, I was looking into the Scarlett 2i2 myself, and a lot of people recommend it because of it's solid built casing and great preamps.
Some other suggestions for interfaces with 2 inputs in the same price range as the scarlett 2i2:
- Presonus Audiobox USB
- Tascam US 122/US 144
- M-audio Mobile Pre
But I think Scarlett comes out as the best choice.
Good luck choosing your interface!