I admit to being a fan of natural rather than synthetic when it comes to base layers, but I’ll also admit to sometimes finding merino just a little too toasty especially riding in warm conditions.
Another fabric worth considering is bamboo, which is what Trail 42 have opted to use for their new tech tee (blended with organic cotton). You get all the benefits of natural fibres (breathable, soft and therefore comfortable next to skin, and most importantly not stinky) but I definitely found this a little cooler than merino. Come winter time and that might not be an advantage but then you’ll probably grab another layer anyway.
The fit is good, I’ve got a women’s size 10 and it’s fairly fitted but not overly so – if you prefer a baggier riding jersey I’d opt for a size larger. It is a pretty pleasant colour too with contrast stitching a nice touch – a good alternative to all the pink that still seems to be out there. Has repeatedly washed well at 30° and so far, seems to be made of pretty durable stuff. Win!