I got the Mini less than 30 days after I purchased the TWSBI Diamond 580. It's that good! One reason why I got the Mini was I felt that the 580 was a little too big for my tiny lady hands. THe Mini is a good fit for my hands. I can write with and without posting the cap. The F nib is a nice width that mimics the width of 0.7 gel ink pens that I used to like. The Mini alike the 580 has a plastic barrel and cap. However the Mini has 3 different colours. It comes in clear, classic and black. Minie is the clear version because Straits Pen didn't carry any other model.
In all ways, the Mini is a smaller version of the Diamond 580, other than a difference in physical size and ink capacity. Though the Mini's ink capacity is slightly less than the 580, it makes up it in the pocketability. I love my TWSBI Mini.
Portable / pocketable
Large ink capacity
Made of plastic, lightweight
Less ink capacity compared to the TWSBI 580
Made of plastic (if you are against that)