You can swap between both wheel standards, on both bikes.

You want the Straggler, trust me. Its geo is designed for offroad, the Midnight Special is designed for on road. I'm not saying you can't ride the Midnight Special on gravel but the steering will be slightly twitchier. Straggler geo is made for gravel.

And all that being said there are even better gravel bikes out there. Just depends on what material you want, and what your budget is. For steel and reasonably priced I'd put the All-City Space Horse before both Surly offerings you mentioned. For more money I'd go with the All-City Cosmic Stallion, Macho Man, or Gorilla Monsoon.