Size and Weight. With cartridge, the Platinum weights 11.3 grams capped, and 9.6 grams uncapped. With cartridge, the Sailor weighs 8 grams capped, and 6.4 grams uncapped. They are close in length: the Platinum is 17.5 cm long capped, or about 6 7/8 inches, and the Sailor is 18 cm long capped, or about 7 inches.
Use and Comfort. I prefer the Platinum desk pen for writing. I love a lightweight pen, but the Sailor, at 6.4 grams uncapped, is too light for me to find comfortable for long. The Platinum weighs more, and because that weight is in the section the pen feels more balanced. In addition, the section of the Platinum is longer and wider, which is great. Here is a photo showing the sections, with the Sailor on top and the Platinum below.
Nib. Again I prefer the Platinum. Both nibs have approximately the same extra-fine width. The Platinum nib was extremely smooth out of the box, and the Platinum desk pen just feels like the better writer.
Keep in mind that Japanese nibs are finer than European nibs. That means that Japanese fine and extra-fine nibs have a very small sweet spot, so they may not suit everyone. But the price point of these desk pens is so reasonable that you can decide for yourself.
Looks and Design. Both follow the classic desk pen design, albeit in basic black plastic. The Sailor is a bit longer and has a white dot at the end which makes it easy to pick out in the pen cup.
I did prefer the Sailor’s cap, which has a tab inside to close with a definite click. The Platinum cap has no tab, so you have to just push the cap until it stops.
Filling System. Both are cartridge/converter pens. I use them with cartridges, because I don’t use them often, but adding the standard Platinum or Sailor converter would allow you to use any ink with either pen.
The cartridges are proprietary so I’ll talk about the inks. Platinum Carbon Black is completely waterproof and Platinum says it is lightproof, which certainly makes it the better choice for artists. Sailor Kiwa-guro, on the other hand, is only water-resistant, to a level comparable to Pelikan Brilliant Black — some dye will wash off but the core remains legible. Kiwa-guro has a darker black color and seems to have a slightly wetter flow.
Both inks performed flawlessly. I ran a little test this time: I left both pens unused for three days to see if I would have trouble with either. No issues: both pens started up immediately.
Build Quality. Both pens are sturdy.
Conclusion. There is no question that I prefer writing with the Platinum. The pen felt much more comfortable in my hand, and I like the nib better, too. So that’s what I would recommend. It is a good way to use a pigment ink without worrying about more costly pens.