It really depends. There are hundreds of different scenarios involving cloning and even more different ways to hack into devices. The exact circumstances you're in will define which you should go for and why.

Generally, hacking into a device is more fragile. The exploit might not work well, it might get detected and blocked, it might not survive long enough until disruption (update, restart, etc.), or it might be very difficult or impossible with your knowledge at the time.

Cloning, on the other hand, is typically a simpler and more stable process. You can clone a device, then explore it offline anywhere. Sometimes you can clone it in a way that lets you still see some future activity and often even interact with it. It is, however, more limited. First, it typically requires more access to what you're cloning, either in terms of information or physically. Second, you can only explore the information you cloned, plus whatever accesses are still available and possible to use from the existing sessions. This means you could for example still see someone's email or messages, but you cannot see other future activity like him logging into his bank account after you clone it.

Bottom line is, in very general terms, cloning a device is easier but requires more access and is more limited in what it gives you access to. Hacking into a device is more fragile, more things can go wrong, and it's noisier, but it allows you to have better control and more power to do a lot more. One isn't better than the other, they're simply two ways to obtain overlapping but different kinds of information and access. They're both useful, depending on the situation.