I'd be cautious. If your finances are so tight that the £15 month a difference in quotes is making the difference in having the dog and not, then I'd say no, very reluctantly. Even if you have pet insurance, every claim has an excess, and it's surprising how vet visits can rack up that are not worth claiming for. In the past year, with our three very healthy dogs, we've dealt with a dislocated tail, a severe gastic upset and a sprained shoulder, most of which were only just over or on the excess charge, so we didn't claim for them. Let's also not forget the importance of having a bit of spare money for classes (essential in my view), plus things like incidentals like replacing rugs after accidents and even the extra cleaning costs associated with a dog.
I'm not trying to put you off - we are besotted with ours - but it is important to have a really hard look at the financial side of it . I know it's very easy to be swayed by a wagging tail, but those big brown eyes won't pay the bills. Far, far better to be a responsible owner and not have one than have one and end up in debt, worried, or even worse, reluctant to take your dog to the vet because you are worried about the cost. That would be a nightmare scenario.
Money is regrettably a VERY good reason not to have a dog - financial problems are one of the major causes of rehoming. With small children, I'm sure by now you know how often you get one of those months - suddenly everyone needs new shoes, or there's a run of birthday parties. Do you really want to be so tight on your budget that you have to choose between shoes and vaccinations? Don't forget insurance WON'T usually cover for things like dental work, definitely won't cover for vaccinations, worming etc and often won't cover for some behavioural consults, or even things like hydrotherapy (in some cases) or other complementary therapies for dogs. Not saying you need this now, but you should be thinking that the dog will be with you for probably another ten years and in their old age, that's when they get expensive (which is why the insurance goes up).
When youngest DC goes to school, will you be going to work? If you do, then you need to factor in costs of dog walker/sitter as well, plus holiday arrangements for the dog. If you still want to do a traditional week away in the sun in August, the cost of the dog's kennel fees could be equivalent to the cost of taking another person on holiday! And the good kennels get booked up months in advance.
Please, please, do think very hard about this.