The 6X-8 heads would give you about 3/4 to 1 point less compression on a 400 (about 8:1) than the #94 heads (about 8.75:1 to 9:1).
Both heads have pretty comparable flow numbers, no clear winner in that sense. I would suggest upgrading the #94 heads to 2.11"/1.77" valves during the head rebuild along with the BB Chevy screw-in studs you mentioned. The new larger diameter seats will be cut into 'new' material, putting the valves where they should be in the chambers and not sunken in like heads with a few valve jobs.
For a 400 engine the #94 head is a better choice since it will put your compression ratio up where it needs to be for best performance. If you were building a 455 the 6X-8 would be a good choice, save them for the future when you might decide to add a stroker crank to the 400 to up the cubes to 455 and beyond.
