The cut varies, I spend a lot of time riding in my Planet X long sleeve but only use the short sleeve as an under layer as it’s got weird arms. Long body, slim fit, and fairly long but again slim sleeves mean the short sleeve is rarely seen by others. It does the job very well as a thermal, it seems like an under jersey cut (I know, I should expect this).
Merino 150 here, really nice. Have been wearing it for about 7 or 8 days with only one wash, due to excess sweating and has been fine. I would consider 120 for 25+ degree days (not that there are many of them here)/
I’ve got a couple of the cheapy base layers and reckon they’re ok for twenty quid a pop. The back isn’t merino but that’s fine, I tend to wear mine as a jersey. I’ve got a large and reckon the cut is fine, reasonably snug but not mega tight. The arms on the short sleeve ones are quite short.