I'm looking at a September hike of the Border Route Trail and temps are falling then. Avg low by the end of the month is 35. I'm trying to decide synthetic or merino base layer. I'd like to hear your thoughts. I have a stack of short sleeve Under Armor/Columbia tech fabric shirts so I'd have to acquire a merino one. I looking at Ibex Woolie 150 crew or Indie Hoodie as examples.