Heads Studs vs. Head Bolts

The studs provide more consistent clamping force. Less variation between each fastener is another way to look at it.

This is what I do if space is tight for studs. Usually it is only some studs, not all of them. I put the offending studs in the heads with a cloths peg around the top of the stud to stop it hanging out the bottom of the head during installation.