It sounds like you have them in series...output of the Rosetta into the MOTU. If so, you will not be able to hear and compare them properly. To do that, you will need to run them in parallel, and switch back and forth between them. To really be able to compare them may involve other things like the type of sound being used to compare them, the quality of the other gear involved, and other variables. If your signal path has a weak link (monitors, pre-amp, mixer, etc) it may mask the difference. Try running them in parallel and comparing. If there is no difference you can always buy a 2 thousand dollar pre, a similarly priced set of monitors, a goooood microphone, and record numerous tracks, mix and master them, and you will possibly hear a significant difference. Or you can take advice from others in the business who have used all types of gear and can explain these differences. I've done a little of both. I'm not pretending to have vast knowledge of this stuff. In the past three years I've been doing this I've learned two things. 1- and the most important is that you don't let the salesman at the music supply outlet tell you what you need. Two, even highly experienced recording pros will have different opinions on what is best and what stinks, you just have to read, try things, and try to learn what works, and what doesn't. Depending on the complexity of the sounds you intend to record, the MOTU may be sufficient, or you may need higher end converters...which means more expense.
I hate Backorders. 
