Depends on what you will be doing with the machine. If you just want it for piecing or general straight line sewing, you can't go wrong with either. If you plan to FMQ, then the 15-91 would be best mainly because of the throat space. My 15-91 will FMQ anything I throw at it with no tension issues, etc. I have several vintage (FW, 201, 328, 66, and more) and two modern Janome machines, but my 15-91 is by far my favorite.
Good luck, let us know what you decide!