So am going to order about 20 prints on line at a size of 18 by 12 inches.

The site I will be ordering from has an upload limit of 5mb per file, the thing I want to know what plays a bigger role on the overall quality of the printed picture, "image quality" or "D.P.I." ?

for example which would produce better results,
200 DPI at 12 image quality
or
250 DPI at 10 image quality.

My photos are taken with a canon 60D (18 mega pixels) so really can't exceed about 275-300 DPI with out having to enlarge the image it self in PS, but what would be the optimal DPI to image quality ratio in your opinion at 18x12 inches.

thanks a lot for any help, I really do know very little about printing. (will be my second time printing my images)