Ehh... Missiles are faster. I've tried Screw Attack on Ridley, and it just takes forever. So I use my missiles. The damage doesn't matter all that much with the E-Tank in the room.
I use bombs on Kraid. I could never beat him with missles. If the Screw Attack takes "awhile" I wouldn't do it because bombs are faster in the first place and the E-Tank in the room makes the point concerning damage moot.
I seem to remember simply grabbing the e-tank in his room putting me up to 2 and just bombing away. I don't think he ever came close to killing me because I had the varia by then.