The AAC Element 2 is very quiet, rated up to .22 mag and .17 HMR, has very little first-round pop, and has stainless steel baffles. But because of its baffle design it requires a tool to take it apart and to push out the baffles.
The new stainless Surefire SF Ryder is intriguing, but I've only played with the aluminum version and I've shot neither one.
The Tactical Solutions Axiom has titanium baffles and uses the same super-quiet baffle design as the Element 2, but it has a sleeve around it to aid in disassembly, kind of like the Sparrow except it's one piece with a small slit down one side. It seems like an awesome design that combines the Sparrow's ease of disassembly with the Element 2's and Spectre's low first round pop. But unfortunately I've never shot one or taken one apart when it was dirty.
I've got 3 NFA items I'm planning on getting in the near future (two of them are just waiting on the Form 3s to clear), but I plan to get a .22 can at some point. And when I do, it will be a toss-up between the Spectre, the Ryder, and the Axiom. The Spectre is known to be an awesome can, but by that time the Ryder will have enough reviews out there that I should have enough information to make an informed decision. The Axiom would be a gamble; in theory it's the perfect all-around mix of disassembly, quietness, low FRP, durability and weight, but it's not popular enough for there to be enough objective reviews out there.