Maybe this will be helpful - I have two bikes that I use for gravel: a Kona Rove ST, and a Surly Cross Check(the "OG gravel bike" according to Surly
. The Kona is 650B x 47(WTB Ventures) and the Surly is 700c x 41 (the stock Knards). The Cross Check is very similar to the Straggler except for the brakes, while the Rove is a
little bit closer to the Midnight Special (if only because of the tire size). Both steel.
I think it comes down to the tire size. I love them both and have a blast on both (on the dirt and pavement) but they are decidedly different feeling due to the tire size. The 650B x 47 is definitely more cushy. But, the 700c is super fun too, and fast. When thinking about which to ride, I tend to grab the 650B bike if I think it's going to get a little more technical. But either one of those Surlys would be great IMO, and you wouldn't need to upgrade any time soon (or ever...).
EDIT: Ahhh--I forgot, the Straggler is also now available as 650B. Duh. Well, then if you're going for 650B, it'll be a tough choice between the two. But the Midnight Special has more tire clearance--that may be the deal breaker there...