I've never kept either. I prefer the look of the Sebae but i've only ever seen Argentus for sale. There is a smaller Mono called Kottelati that looks like Argentus but i've never knowingly seen that for sale either.
I saw six large M. sebae in a 120 gallon and to this day,I'm still stunned. THAT'S how great they look at 6-7" long...even younger 4-5" are impressive. Elegant fish..graceful swimmers. Almost imperial looking.