YS for sure, the gameplay is far too superior and refined in almost every way.
I like Trails, it has its own merit and I don't think there's another jrpg series who have that strong world building, but outside of that and the continued plot it's not that fun to play. There are some pretty cool character (like, really, I wish every jrpg have a charismatic protagonist like Estelle), but most of the time people talk too much and say something futile or obvious. The battle system is not bad, but it's just too slow, I don't know how I could have handled Cold Steel without the Turbo Mode. Overall it's a pretty good series, it's just that YS is way more attractive to me with its excellent battle system and addictive gameplay, nice exploration and environments, some metroidvania mechanics, cool dungeons ecc.