I can appreciate that. As a recovering engineer, I drive my wife nuts with all the analysis machinations I go thru on most of our big purchases.
Just like cars, boats, tools, speakers, etc ... there is no perfect jack of all trades. It’s why most of us have multiple cookers, or you make intelligent compromises.
You can/will make good smoked meat on either (let alone all the other styles/fuels - electric, propane, charcoal, kamados, Offsets, etc etc). Since you seem to have settled on pellet one way or the other, it does simplify the pro/con list.
You will not fit a whole packer brisket on a vertical, maybe not even an uncut rack of ribs depending on the model ... if that’s important you go horizontal.
You will not sear or grill in a vertical ... if that’s important you go horizontal.
Usually think you will get more space for the money with a vertical.
A vertical might be somewhat more fuel efficient (though maybe not by much, not sure)
Others may weigh in with any other factors I’ve missed.
Good luck ... at some point you just close your eyes, throw the dart, and go with it. You’ll be happy.