These machines are very different.
The 1638X is a dual head printer with no cutter. It is almost 4 times faster than the VSI series at the lowest resolution. If you are doing a lot of banners, posters and signs, it's a great production machine.
The Roland VS640I is a single head machine with a cutter. If you are doing a lot of decals and contour cuts, it's a great machine. Both use the same head so the resolution is the same but the second head on the Mutoh makes it faster. The Roland also offers the white and metallic ink whereas the Mutoh doesn't. White and Metallic ink is very troublesome but if you need it, Roland is the way to go between the two. I would recommend the Mimaki JV150 or 300 over the Roland for that ink set however.
Roland drops you as soon as the warranty is over so having a good dealer with a tech is key. Mutoh supports their machines directly and through dealers and, in my opinion, has superior tech support to Roland. It may be because Roland is a huge company and Mutoh is much smaller. I have had customers with 10 year old machine call Mutoh and they helped them over the phone. Roland does not do that.
Overall I recommend the Mutoh but it all depends on what your needs are in the end.