Of those you listed, I'd probably go for the XM-18. The SnG is too thin for me, personally, and the ER is a nice knife but has to be sent to Italy for any repairs.
As to thick vs. thin knives, I like my Devastator tanto. Thick tip for strength, and thin belly for slicing. Of course, its more than $400. While I can see (and have made) the argument for the Spyderco vs. Custom, in the end it comes down to how you want to spend your money. The Manix is a fine knife, but you probably don't need anything more than an Opinel for most cutting chores. Does that mean you overpaid for the Manix? No. Its just a matter of perspective.
I mean, I like to go hiking with a Mastersmith Hunter on my hip. I could use a Mora for all the stuff I do, and the Mora would pass with flying colors. I just PREFER to have the MS Hunter on me. I like knowing the person who made my knife, and I won't EVER try to nickel-and-dime someone down on thier handmade crafts.