Hi Carl (almost a homonymous!) and welcome to the forum!
I split the discussion since that other one is only for reviews while it's better to open a new topic when you need specific information.
Back to the main topic, here on this site, as on UM site, it's listed that calling the US from Zone 1 costs 0.35+0.25 on the first minute and 0.39 for the following minutes. The USA is on Zone 1 as
destination, while it's on Zone 7 as
origin for calls. That means that UM is good if you go to Europe and call the States, while it's bad if you go to the US and call anywhere.
The rest you wrote is correct, it's never easy which one to choose, surely UM is a good product and we all know it works well, Yakie is something new and by now no one has received its sim-card yet, so maybe it's better to wait a bit, if you're not in a hurry.