It would be surprising if you could hear any kind of substantial difference between two studio-grade DACs made in the last 20-odd years. The Mytek seems to be using internal ASRC so may not be fond of >0 dBFS digital levels (intersample-overs), but given that it uses a DAC with a 120 dB dynamic range (presumably AK4396 or AK4395), giving up 2-3 dB on top should not be a major concern, especially in a studio where you are not generally dealing with heavily brickwalled material anyway and sneaking in a bit of digital attenuation should not be too difficult even if so.
Spec wise, the Focusrite with its midrange level 110 dB A/D and D/A, 0.001% (-110 dB) THD+N and +16 dBu fixed output (vs. a maximum of +29 dBu for the Mytek) is no doubt the weaker of the two, if still good enough for practical use. (You would probably want a power amplifier or monitors with adjustable input level, so that +16 dBu line-level signal can be matched to 110 dB SPL or less.)
What strikes me as most concerning isn't even related to the DAC but in fact the mediocre -90 dBu input noise figure for the line-ins, which means you'd have to run your preceding mixer or whatnot rather hot - even at +20 dBu in, you can can get at most 107 dB of dynamic range out of the ADC, and a loopback measurement from DAC out to line-in should yield around 104.5 dB, which while adequate is hardly awe-inspiring these days. This may not be a concern if the rest of your equipment is at big studio level, but with common inexpensive home studio gear it may prove an inconvenience.
Basically, I have little doubt that you can live with both. Newer gear may shave off the odd millisecond in latency and spare you the inconvenience associated with still operating Firewire these days, but sound quality should not be a dominant concern assuming everything is working properly. Some monitors with limited level input adjustment range may require a monitor controller for best results with the Focusrite; the Mytek with its internal jumpers, pot and analog adjustments is more flexible in terms of analog levels.