Lets say a Rocket c56-57 and SF c 59

These are two of the most respected brands out there, and whichever you choose you will have a decent enough machine.  I have the Olympia, which I cannot fault for the size and weight, and feel I can rely on for travel, but have only tried the Hermes once, which I found nondescript and 'average'.  Too small an experience to be objective - but I can say that the Olympia is very unlikely to disappoint.

If you already prefer the aesthetics of the SF, then the Olympia is the right choice. I own several examples of both models, and although I actually prefer the aesthetics of the Hermes, the Olympia is a more robust design and its action feels more like that of a full-size portable. As Beak stated, it's not like you'll be disappointed by either machine, but one model is more likely to check off more boxes than the other when you know exactly what you want.

I'd have to check my typewriter spreadsheets later on to verify it, but I think the SF is a little heavier. That shouldn't be an issue unless you plan to travel with it. In terms of travelling with the original set-up (I would advise against it), it's worth noting that the travel case (lid) for the Hermes is metal compared to the plastic lid for the Olympia (there were actually three case options available for the SF, two clamshell-type soft cases and the hard plastic integrated cover).
 

Lap use:
Unless you are going to use a solid surface on your lap, such as a tray or a solid board, you may find problems with any machine which does not have its own in-built solid baseplate - putting a TW on a soft support such as a cushion may cause faults in the action which will be annoying.