This depends on your needs. The Scarlett Solo has only one preamp, whereas the Scarlett 2i2 has two. The Scarlett Solo does have another input for instruments, such as an electric guitar, otherwise known as a line input.  But, if you need an interface that will allow you to plug in two microphones, two instruments, or one of each, then the Scarlett 2i2 is the best option. If you only need one mic input and one instrument input, then the Scarlett Solo is for you.