it would be a better idea to sack guitar rig instead...
honestly, the quality difference between that and even the Scope 'classic' guitar amp/fx emultations (Celmo's - let alone Dynatube...) are by magnitudes larger than the subtle difference between those 2 cards

you can try it yourself with Celmo's free versions (or his demos)
just switch off the 'overused' FX in guitar rig and compare the pure amp and speaker settings...
it's easier that way and more focussed - imho it would be hard for Guitar Rig to compete against the full load of Scope FX anyway.
don't misunderstand this as a diss of Guitar Rig - it's just a very good example about the 'special' sound of Scope.
It's not immediately obvious - I also found the demo of guitar rig pretty impressive - it DOES sound good, no question - and the presets I tested were convincing, too.
but then I did the A/B thing, picking similiar setups from Celmo's Bassamp sim (same engine as the guitar version and guitar presets, too).
Only then I realized how good the Bassamp really was
a 'sound' is not only in a converter or the 4th decimal of a thd figure...
even if you improve the (already great) converters of an RME significantly, devices like Guitar Rig will still sound the same and NEVER even remotely as punchy and fat as Scope devices - it's in the algorithm of the sound engine and that runs, well... on a Scope card only
I've tried to be honest from a (lousy though) instrumentalist's viewpoint (like in a shop when you compare different amps and cabinets) - my taste may be different from yours, so don't take this for dogma.
But you WILL be able to hear the difference (with comparable setups) - whichone you prefer is of course entirely up to you.
btw an RME is first choice to connect a Scope (via Adat) to non-compatible hardware or OS versions, like Mac OSX
cheers, Tom