All my HH lifa base layers are Polyprop, they’re great as thermals the only downside it they stink after 1 ride.
However my Howies Merino gets wet and stays wet, so if you stop you get cold quickly. On the up side it doesn’t smell as much.
I don’t have any Polyester base layers just road jerseys, so don’t have an opinion on their thermal properties.
Merino is great for walking and maybe touring where you’re not going full pelt but not mcuh cop for fast rides.
That’s in my experience.