It depends on your circumstances and the terms of the lease.
Terminating the lease provides the cleanest exit. Normally, you have no continuing liabilities to the landlord after the lease is terminated.
Assigning the lease is generally less clear-cut. Depending on the terms of the lease, you may well be left with some liabilities. You often end up either liable for all future payments owed by any future tenants, or having to guarantee some or all of the next tenant's payments.
The process of assigning a lease can be more costly and time-consuming than terminating a lease. As well as finding the new tenant, you will need to negotiate with both the new tenant and the landlord.
If you lease has less than two years to run, it is often preferable to terminate the lease to avoid the costs and risks of assigning it. However, assigning a lease is usually the best way of realising any value the lease has.
While sub-letting can be a way of improving your cashflow, it is not a satisfactory way of getting out of a lease. The original lease remains in force, and you retain all your liabilities.
