The XS-600 with a 100, would still be very similar to the Tornado. The hull design between the two is very similar, although the XS has a newer 3-rail design based on that of the larger Tornados. The Tornado 6m as I've said has a 2-rail design. There isn't much difference between each but I have noticed having driven them back to back that the XS lifts higher out of the water when planing. This makes her a small bit faster and more efficient. What's more noticeable though is that the XS, grips far better in turns. The Tornado on the other hand has the advantage of being lighter than the XS but not by much.
To be honest I think the XS 600 with 115 Optimax is probably the best package, ours is a bit overpowered at 150, it's extra power that most people never use, but it is nice to have.
She would go well with a F100, but the lighter and more powerful 115 Opti would be ideal.
A friend has a Tornado 6m / 135 Optimax and we go out together regularly. There is very little between the 2 boats. Ours is 5-6 kts faster but we don't know if its down to power or hull? Both excellent boats though and you won't go far wrong with either.
If it was me I'd stick the F100 on a an XS-600, you could always change the engine in the future and you've got the piece of mind that you're getting a new boat with 3 year warranty.