If your daughters want a dog that badly, you might look into fostering a rescue pet (lots of resources online - just look up fostering a dog). Or you might talk to them about volunteering at a shelter before agreeing to welcome a dog into your home.
I agree with the others - your teens will likely be dating, going off to college, taking on more school challenges and activities, and you'll be stuck with the dog (unless, of course, your girls are home-schooled, or there's some other reason why they will be available for extended periods of time to care for the dog). A puppy requires a tremendous amount of consistent training and care. There are vet bills (even for a healthy puppy, there are immunizations, and eventually spaying or neutering). You might ask a local vet what a typical puppy costs.
You could also visit a shelter together and ask about what is expected when getting a new pet.
If your girls love pets, and since they're teens, they should take the initiative themselves. Let them find out the costs associated with a new puppy, raise the money, start working at a dog rescue or doggy day care, and start researching dog breeds. A lot depends on your situation: do you live in a small apartment or large house? does your home have lots of stairs? does your home have a fenced in yard or do you live in a city condo? does your neighborhood or city or homeowners association or landlord prohibit certain breeds (you'd be surprised what dogs are being added to the growing list of restricted breeds)? Let your girls look into that. For example, if someone lives in a second floor apartment with no elevator, it would be challenging to get a little dachshund. That dog may have difficulties with stairs and you're going to need to carry it up and down every time it needs to go out. Or if you have a fairly small home, you don't want to get a dog that's going to be huge and needs space to run. If your older girl already knows or hopes that she will be going to a college in another state, or a couple hours away from home, or a plane flight away, then there's a short time before college trips will be happening. And traveling with a dog can be challenging. There are hotel fees and boarding fees to consider. If you go to a ski cabin or a beach house or grandma's apartment every year, will you want to take the dog, or even be able to take the dog (look at breed restrictions or pet restrictions if you travel frequently to one location), or are you willing and able to hire a pet sitter or board the dog?
So, I think you could present them with a list of questions, including their future plans, and costs, and breed restrictions, and travel plans, and puppy training requirements, and when they've thoroughly researched everything, make a decision together.
Get them a different birthday present unless you have decided you want a puppy for yourself, and then call it theirs but be prepared to have all the dog responsibilities on your own for a long time to come.