The M-5 is a good sounding, clean, clear sounding mic pre. It will be faithful to the signal you put into the unit without embellishment. If you're looking for a unit that will make you go "wow" everytime you plug something into it, then this probably isn't the box for you... if you're looking for detail and clarity, you will be able to achieve it with ease with the M-5.
As for RME stuff... I have never heard any RME converter I thought was worth a damn. I find them rather small and nasal sounding in comparison to things like Apogees and Crane Song's, Lavry's or even the Lynx cards... but I have a feeling those things are probably more expensive than the RME stuff.
These are just one user's opinions... they may or may not be relevant to your reality... in other words, "your mileage may vary"