Go threadless 1 1/8 with a nicer narrow profile type stem. It's going to be a modern steel bike, not a vintage one. Will it be lugged? If so then look for a lugged threadless stem, can look really nice. If its not going to be lugged then just go with any option you like.

Threadless gives you some better headset options and you may want to swap forks in the future for front loaded touring or light CF... whatever.

Quill can also feel flexy.