Caching and putting the whole OS on it: Which is better?

Is it better to use it just for caching or will it be better to install the OS on this thing?

this is just a 32GB drive so I would say it is best for caching. That said you can put the OS on it if you are OK with being limited to 32GB of starage for youer system drive. 

honestly thoug if you are going to install the full OS on the SSD go for an ultra plus or extreme. they are both not much more expensive for much higher capacities.