Now i haven't had the chance to use Large Arachnid But i used soulstone miner often enough. It does make a great scheme runner durable as well if you want to burn the stones for prevention. That said i personally dont feel like it will brawl nearly as well as the large arachnid being able to eat scheme markers and scheme markers. while the Same SS cost they are hard to compare. but i will often bring soulstone miner even if there is no scheme needed for him as From below lets me place him in position that can put on a lot of pressure.
For the Metal Gamin vs Steam Arachnid i dont think i would really start with either in my lists but i will gladly summon in More spiders over gamin with a Mech Rider simply because higher speed, higher defense, evasion, and latch on. Also with in my expierance armor 1 And armor 2 not being a huge difference when you are summoned in with only 2 wounds remaining anyways.