Statistically, Shopee only has about a 10% advantage on Ruten in terms of visitors to the websites, though this seems to be on the increase. Mobile app traffic use isn’t reported, but Shopee has far more installs.
I use my phone for a lot, but I find it much better to shop through a full website so I can do real comparisons and research. It’s not easy to multitask or get a broad view of a product on mobile. Too easy for me to make mistakes like this:
Anyway, smart sellers sell on all platforms with significant traffic. Also, Taiwan has a lot of old people, by which I assume you mean over 40. For me, I buy a lot of second-hand or custom made parts, and Ruten seems to have more second hand products. I don’t know about the flexibility with custom made items on Shopee, but it’s fairly easy on Ruten. Also, Shopee is too red.
A service like OLX would be useful here. I used that a lot during my time in the Philippines. It has expanded, but not to Taiwan. If there’s anything similar I haven’t found it. Maybe FB groups serve the same purpose.