my choice, because it is more positive i.e. "it is only a question of time before we accomplish our task to give you full satisfaction"
whereas IF is conditional and thus somewhat questionable

Although both are fine grammatically, and have the same meaning, I think "when" is somewhat more positive. If you use "if" then you admit: it can happen that the client will not satisfied. If you use "when" then you may argue that it is only a question of time that the client will be satisfied.
You can also use "if and when" but it is very heavy-going.

I cannot see how you can use "when" unless you change the tense of the verb. We will only be satisfied when you are
We are only satisfied if you are.

:)

As marketing slogans go, I think this one is back to front - starts talking about the supplier first, then the customer. Also a bit short of pith - it's too wordy as it stands. And I think the sense is not if or when, but until.
So I would do it this way. Possibly "Only when you're satisfied arer we satisfied."

English term (edited): We are only satisfied *** you are too.

ProZ is the site where you come with a simple question and go away with a headache!

OK, let’s see what you have:
Title: Mutual success [I thought this was all about mutual satisfaction...]
Last statement:
"We are only satisfied *** you are too."
Your question: Which word would you use in this typical marketing slogan and why?

First of all, drop that ‘too’.
Most people seem to agree that ‘when’ is more positive than ‘if’--so let’s use ‘when’ in these examples.

One question to answer is, do you want ‘satisfied’ once or twice, e.g.
- We are only satisfied when you are.
or
- We are only satisfied when you are satisfied.

Do you want ‘We are’ or a contracted ‘We’re’?
- We’re only satisfied when you are. [A contracted form would be wrong if ‘you are’ is not followed by satisfied.]
or
- We’re only satisfied when you’re satisfied.

The strong tendency is to place ‘only’ right before the verb, but grammarians insist that the best placement is precisely before the words intended to be limited.

So your ‘grammatically correct’ options are:
- We are satisfied only when you are.
- We are satisfied only when you are satisfied.
- We’re satisfied only when you are.
[The fourth option didn’t sound good to me.]

‘If’ is still valid but, as we’ve said, not positive enough.
In any case, I found a few:
- We are satisfied only if you are.

The present tenses describe a habit: this is the sort of promise we make to all our customers. But we can look at it as the future promise we make to the customer as he walks into our shop. We can then use the future tense in our main clause (but not in the if- or when- clause).
- We will only be satisfied when you are (satisfied).
- We’ll only be satisfied when you are (satisfied).
- We will be satisfied only when you are (satisfied).
- We’ll be satisfied only when you are.

But you don’t have to restrict yourself to if’s and when’s. Google will always provide a few more variations.
There’s also ‘until’:
- We’re never satisfied until you are. (49 Google hits)
- We’ll never be satisfied until you are.
And ‘unless’:
- We will never be satisfied unless you are.
- We cannot be satisfied unless you are satisfied.
Or a different order in the you’s and we’s:
- If you're not satisfied, we're not satisfied.

Now if you don’t want to be so laconic and decide to show some enthusiasm, here are a couple of variations (always found in Google):
- We will not be satisfied unless you are completely overjoyed with your purchase.
Or from this Italian Walking & Cycling Agency:
- We are satisfied only when You are very happy, absolutely satisfied and completely
relaxed!!!

How’s the headache?

Now all of the above are valid, used by people on their sites, though admittedly not all of them in Anglo-Saxon countries.
Which one would I use? Well, I like my only’s correctly positioned, so:

- We’re satisfied only when you are.

But people are more likely to say:

- We’re only satisfied when you are.

But then I’ve got a headache too after all this.

I would use the two prepositions together, this stresses the fact that it is essential (if) and will happen sooner or later (when).

English term (edited): We are only satisfied *** you are too

We are only satisfied once our clients are satisfied.
If, for any reason, you are not satisfied...

"When" is rarely used in this context.