What sort of cache are we talking, SLOG or L2ARC?
The question you should be asking yourself is, do you really need either SLOG or L2ARC?
FreeNAS likes RAM, the more you add the better the system will perform. If you're thinking L2ARC, max out your RAM and watch your ARC stats/ratio, that will tell you if you need an L2ARC. An L2ARC will also utilize RAM, which is why you should max it out first. If you're doing synchronous writes, such as a NFS datastore for ESXi then use it as a SLOG to improve write performance. If you're not doing this, then a SLOG is going to do nothing to improve performance.
Adding a 'cache' device for the hell of it can be detrimental to your systems performance.