My understanding is that the 5600 has better wireless capabilities than the D5500, slightly heavier and better at power management, but the rest is the same, for a slight increase in price.
I have figured this is not that important for me and would take the lower price instead, so I ordered a D5500 + 18-140 kit.
Also (separately) a 35/1.8 DX, extra battery and an SDXC card.