I am of the assumption that you are referring to the RJ-900 vs the F Series.
I have mixed feelings ons the F Series as a friend of mine beta tested one a couple of years back and was not impressed. He could also never achieve a dense black which is another thread altogether. A few searches will yield that there have been numerous issues surrounding this printer. His 900's work like a champ and have very few, if any problems. I have a few Epson 44" printers and they have performed "fairly" flawlessly other than the constant head cleaning and lack of being able to service it efficiently.
With that being said, our 1638WX is a beast and can handle anything you throw at it. Automatic head cleanings, ease of maintenance and being able to let it print all night long are great features. We just finished printing 1,200 singlets and had not one issue to complain about. I wouldn't hesitate to purchase another Mutoh.