I've been researching for a new gooseneck for the last few months. I can't answer your questions specifically, but from what I've gathered, and whats guiding my shopping .....
1 - Triple axles are a PIA and do eat tires, I can't say how fast. I feel like this is why they are often sold as well.
2 - People dislike the low trailers because they often drag the minute you get off pavement.
The guy I was camping next to at KoH was damn near about to cut his dovetail off right there from dragging it.... his buddy behind him loved it though ... made the road smoother LOL. So unless you're planning to put a tall camper or monster truck on it I wouldn't bother.
For those reasons I'm shopping for the low deck over type. Like this Diamond C for example:
The deck is over the tires, but every consideration has been taken to keep it as low as possible including plate over the tires vs a thicker wood deck and the top of wood deck is flush with the top of the rails vs a whole other layer over them. Also no dovetail.
Just my $.02