All excellent advice.
First ... get to a large show and try as many machines as you can, with both orientations. Just like regular sewing machines, and long arms (on frames), you need to get and like the feel of the machine.
Consider the space where you are going to set the machine up. Will you have space at the back of the machine for an extra table or extension? If so, a horizontal (i.e. like a domestic) machine could work. Will you have limited space to the back, but ample space on both sides of the machine? Then consider a vertical orientation.
And yes, there are other considerations as well. If you think you will need support, a dealer local to you will be important.
Also consider ... bobbin size, lighting, availability of a stitch regulator (try them with and without), and other features like ease of speed control, ease of needle up-down switch, ease of threading and inserting the bobbin (some of those bobbins are deep inside and may require longer fingers or smaller hands!! So YES ... make sure you try taking the bobbin out and putting it back in), integrated bobbin winder (preferably with a separate motor), and yes ... cost.
I had done all of my homework, tried several machines (6 or so), and did my comparison spreadsheet. In the end, I purchased the Juki sit-down and I love it.