Surely there must already be enough information for you to decide?
Both are mature ladies, 50s or 60s according to reviews.
I'd guess Yna is early 50s, never seen Li.
As you say, information on both is hardly lacking on UKP. 50% of people may prefer a booking with Yna, 50% Li, or more likely the majority would recommend seeing both.
Yna offers a proper, therapeutic massage and a bit more, Li sounds far more focused on the sexual aspects of the massage. Yna's sessions are 90 minutes. Both of their pricings include HE so none of that extras faffing.
If the OP cannot decide just go with whoever is more convenient to travel to. I'd see Li if she was closer to me.