What are the differences between the Camp Plus SI and the Comfort Plus SI mats?

The foam core of both the Comfort Plus SI and Camp Plus SI is identical - the woven fabric of the Camp Plus SI compresses the foam to a slightly greater degree, so we list the Camp Plus as being slightly thinner.
 
The face fabrics are where the two mats differ. 
 
The Comfort Plus SI uses a 30D woven polyester on the base, but a 30D stretch knit polyester on the upper surface. The stretch knit is generally perceived as being more luxurious; it is cooler to the touch in warm weather and warmer in cool weather (much as a flannel bedsheet would be).
 
The Camp Plus uses a 75D woven polyester for both the base and upper surfaces. It will be more durable than the stretch knit of the Comfort Plus.
 
With respect to R-Values: the Camp Mat Plus tests very slightly higher than the Comfort Plus, but the difference is insignificant in practice.
 
The weight of the Comfort Plus is slightly higher, because of the stretch-knit fabric. If you are intending to use either of these mats in a car-camping environment (which is the likely use for both mats), this is a relatively minor factor.
 
In summary, if you are looking for ultimate comfort, the Comfort Plus would be your choice.
If you are looking for much the same level of foam support and insulation, and are less concerned with the tactile feel of the mat, the Camp Plus would be the one to go for.