BT-99 is a heavy-use competition gun, Spartan is a cheap truck gun. There's just no comparison between the two, except that you can put the same shells in them.
I have a 20-year-old BT-99 with hundreds of thousands of rounds through it. I use it regularly. It doesn't look new, but the birds think it is.
All I can say is, trust me, they're not overpriced. No one I've spoken with who owns one has ever thought so. It's by far the least expensive gun in its class.
Hammerless singles are good for one thing and one thing only: American Trap.
For a truck gun, I like the hammer. Simple and safe.