Placing clean sketches into Illustrator...

Im having trouble placing sketches i scanned into photoshop into illustrator. my problem is when i place them into illustrator the resolution is a little choppier than pshop. i tried resizing and changing the resolution but it still seems to be not quite right. i also changed the pshop files to cmyk before placing (i dont know if tht would help). i scanned them in at 150 dpi. any suggestions on how to transfer sketches from pshop to illustrator to look flawless like the scan would be greatly appreciated…

cheers
josh