What expression would be the opposite of "Deal Breaker"?

I understand that "Deal breaker" is an expression used for a feature/characteristic that would make one not go for a deal (or terminate a contract), even if the deal's other features are great.

What expression would be the opposite of "Deal Breaker"? I'm looking for an existing expression that would describe a feature that would make one go for the deal, even if there are other not so great characteristics.

The expression could fill the sentence below:

I know this phone carrier has higher rates and everything, but if they are offering free iPhone to us if we switch, that would be a <expression here>! We would totally go for it.

Solution 1:

That would be the clincher - a fact, argument, or event that settles a matter conclusively.

In OP's case, the matter in question being which deal to sign up for.

EDIT: Since there's been some discussion about "levels of formality" in comments, I'll just say that clincher is far less informal than "we would totally go for it!" in OP's example context, and that a "clinching argument" wouldn't be out of place in the most formal debating society context.

There's also killer feature which "kills off" the competition (cf killer app), but I think this is more popular with ad-men and reviewers than with Joe Public trying to choose between rival products.

Solution 2:

For whatever reason, we normally don't express the concept "there are lots of disadvantages, but this one good thing outweighs them all" in a simple phrase like "deal-maker". We often say that one critical feature seals the deal, but that usually means the other features were still positive, just not overwhelmingly so. (Just now I see FumbleFingers has submitted clincher. That is a nice answer, and serves as the single-word form of "the thing that seals the deal", but it still suffers from the connotation that we had been leaning toward making the deal anyway, and this thing just pushes us over the top.)

Solution 3:

While it is not strictly an opposite, I have heard "no-brainer" used in that context: "Ok, the price is high, but they're giving me an iPhone? That's a no-brainer!" The sense is "I don't have to think about that; of course I'm going to do it".

A less casual phrase would be "automatic win": "This provider, unlike the others, will give me a free iPhone. That's an automatic win."

Solution 4:

As I stated in comment, deal-maker comes to mind most immediately. If you are looking for a single-word answer, look no further than FumbleFingers.

Clincher is to catch as deal-maker is to deal-breaker.