Hi Marlusz,
Functionally the amps are the same except the with the exception of the output tubes. The I880 is a KT88 based amp that's rated at 50wpc. The I2A3 is a 18wpc 2A3 based amp. In terms of sound the I2A3 is more refined with slightly better bass control. The sound stage is slightly bigger and overall it's more euphoric, although far from syrupy, the Melody gear just doesn't go that way. The I880 has a bit more punch and rocks a little more, because of the tubes and increased power. The internal part quality is a bit higher on the I2A3. The I2A3 is much more expensive.
That pretty sums things up. Feel free to PM me with any specific questions.