I have both, and strongly admire and recommend both.  There would be little to choose by way of quality, but there is a difference in feel of the actions.  Neither would be considered 'better' than the other, but you may prefer one to the other.  The SG1 has a smooth and classic action, the Ambassador a solid and dependable action - those who have tried a Hermes 300 would know the distinctive feel.
Both SG1s and Ambassadors come in a range of slightly different models according to date, so I would not feel qualified to give more than this generalization.  In brief, given that the machine is in good order, you could not go wrong with either, IMO.

Really...
Interesting.
I assumed it wouldn't be close at all with the SG1 dominating.

You know since you mention Hermes, for some reason I just haven't liked a couple machines that everyone seems to love.
My Hermes 3000 felt sluggish, and so did my Lettera 32. Like when you squeeze water out of a bottle and it takes a little longer for air to fill in the gap...like the keys weren't immediately ready to be pressed again. More with the Lettera than 3000 but still, I think the action on a Royal QDL was far superior in just about every way.

Then again, action on my 1968 rocket is lighting fast. Maybe I just got a couple bad ones...
Thx beak!

Just picked up an SG1  about 30 minutes ago, but it $20 so even if I decided on the ambassador I would've picked it up anyways, at that price. Posted pic in acquisitions.

Javi wrote:

... the SG3, which is plainly indestructible. In addition the mechanics of the SG3 seem more "normal", mostly in the carriage ...

​I would add that I would much rather have to repair an Olympia than a Hermes! I can't speak to the Ambassador, but all the other Hermes models that I've worked on were the stuff of nightmares.

Javi wrote:

Another comparison: what about the Olivetti Lexicon 80?

NO!!!!!! 
 

The Lexicon 80 is Olivetti´s best seller in Spain ever. The venerable Hispano Olivetti Lexicon 80 (not Lexikon) was everywhere, and it stuck. It´s tough but yeah, Uwe is right to a certain point.

It has nothing to do against a SG1, but c´mon, it´s not that bad! There´s a certain run on early Lexicons which have really annoying keytops because they look nice but are too small and you can easily stick your finger between keys. In fact, there´s as much empty space as keytops, turning the keyboard into a trap fro fingers. And it has an unique feature: that weird recoil which makes typing feel like jumping on a trampoline. You press a key and the machine "lifts" it back, your finger included.

All in all, it´s extremely easy to find here in Spain and competent enough, but it´s no SG1, true. But I wouldn´t say it´s bad, either.

And nope, no paper injector. Never seen one with it, at least, and I´ve seen quite a few because as I said they´re everywhere and if you´re not careful you can trip on one, fall off and lose a couple of teeth.

Sorry, I think I´ve deviated a lot from the OP!