I don't use a Pro 40
or an MR816. However, I do run a Yamaha n12
(which Yamaha/Steinberg stripped down to produce the MR series) alongside a Focusrite Liquid Saffire 56
(the Pro 40's big brother). So, given the close family ties, I can probably offer a pretty reasonable assessment of the two.
And, to be honest, in the same room with the same sources, same mics and the same monitors/headphones, they're very, very close. Neither one is "night and day" better than the other. The pres are similar in terms of the available gain and clarity of tone and the AD and DA are of a similar standard. If I had to give the edge to one unit over the other, I'd hand the prize to the Focusrite but the difference really is minimal. In my experience, the Focusrite is a little more transparent, clearer and sharper and the Focusrite drivers (on Windows - don't know about Mac) are a tad more stable and better performing than the Steinberg ones. But if you were planning on using Cubase as your DAW of choice, I'd then say go with the Steinberg just for the convenience of the Cubase integration. They really are that close.
In either case, if you can't get good results out of one or the other of these, it isn't likely to be the interface that is at fault. You should be able to buy either one with reasonable confidence that - for the price - you're getting a good package that should serve you well.