One of the things to keep an eye on is the "lifetime cover" clause.
With "non-lifetime cover", if your pet develops an illness, that condition is considered a pre-existing condition when renewing, so you'll no longer be covered for it. The premium tends to be cheaper for this type of insurance.
With lifetime cover, the illness will continue to be covered. The premium is more expensive for this type of cover.
It's a bit of a gamble, your decision might depend on the breed of dog (if there are specific illnesses that it's susceptible to that could be expensive to treat).