All early scans measure the baby :) and should show you the heartbeat (you dont usually hear it til further on)
All of the early pregnancy scans on window to the womb will also measure the baby to confirm how far along you are.
Early reassurance scan should measure the baby and give you an estimated due date and let you know how many weeks you are, They wont allow you to hear baby’s heart beat yet until around 16w when you can purchase a heart beat bear at your private scan thats probably the earliest you’ll hear it unfortunately
If you’re going after 8 weeks then it seems you can book either. The viability is usually just to. Check everything is as it should be. They should still measure the baby though and tell you how far along you are. I had ivf so my clinic do my scan. But they measure the baby’s size. And from what I’ve seen of other scans on here they seem to be getting the baby’s size / gestational age.