Offline Chache is used to store your game assets, logic, files, music and all of that.
The C2 System Save and Load feature job is to save data state. ie position of objects, variables and such. Save/Load state doesn't deal with assets.
Since neither overlaps the other domain (ie banana milk shake or steak dinner) neither can really be compared to use one over the other.
I think maybe you want to consider WebStorage instead of offline cache.
Now if that's what we are talking about. I'll give you my opinion. WebStorage. Using Load/Save is great for game states similar to Emulator save states. hwoever Savestates use a lot more storage space. WebStorage however requires only a little overhead and is likely only uses 1/100th the storage size.
I tried using Save/Load once. I cut out most of the level objects and just had a handful to save. Player mostly and the level progress.
The Load/save stored at about 300kb
Where as my custom used about 300bytes
To me. Use Load/Save for mobile devices. When the player leaves the app then Save the State. If the player comes back to the App load the state.... not that I think our Apps can no when they are being shut down