Having already done this, I can say that the Elmers pen was not as precise as the Uchida dye pen. The Uchida had a super fine 'brush' tip and was easy to apply and wipe up. The alcantra is tricky but didn't take more than a few minutes to do with no issues.
Picture is a little blurry, but you get the idea.
The dye pen will not 'leech' out onto clothing and will last longer and resist fading. The Elmers pen may be brighter and add more punch but I would still recommend the Uchida dye pens as my first choice.
Finished seat:
