In Deadman's 90's, you spin on the first hand that touches the ground. Like, if you wind up with your right, you put your right hand down first, then kick up and spin CCW in that manner.
In regular 90's, you use your second hand to spin. If you wind up with your right, you put your right hand down first, then you put your left hand down to get into a handstand. From there, you take off your right hand and you spin on your left hand. Doing this allows you to get more rotations.
Deadman's are probably easier to do quickly, but they get less rotations, so it's a matter of preference then.
see for me i can only do deadman 90's i have been doing it for about 2 months now and there r times where i can't rotate more than 3 times like u need to tuck in ur head in more. whip a bit harder and u need have ur legs striaght up like if u r going into a normal 90 but u don't shift ur body weight to ur right but u remain with ur left (ccw) or right (cw) instead