Actually there are several key differences in the 2 models. I believe for that additional $30 more, you are getting a much better deal

.
Pros:
1. CI320 is small, about 1/3 the size
2. CI320 is passive cooled, meaning it emits zero noise
3. CI320 comes with Intel AC wireless 3160. ID18 does not (unless offered by the vendor and might not be AC standard)
4. CI320 hass a quad core Bay Trail CPU (released 2014). ID18 is a dual core Ivy Bridge CPU (released 2013). For technical differences, please see here -
ARK | Compare Intel® Products
5. CI320 uses half the power consumption than ID18 but more powerful in terms of processing power.
Cons:
1. CI320 can only hold 1 x 2.5" SATA drive. ID18 can hold mSATA and 2.5". But this is purely because of the form factor.
Note: The ID18 has the same CPU as the MSI newly launched mini PC.