What's the best way to scan for photo services?

[Mapidus]

Would you get better quality scanning 600 dpi at 100% or 600 dpi, but with scaling at 200% for an effective 300dpi, but 4 times the area photo (same amount of pixels, just presented differently). Basically what I am wondering is if you give a photo service a 600dpi file will they use all the resolution or do they downsample, in which case giving them a pre-blownup picture might be better. Has anyone ever tried that. I want some blowups (I know quality won't be that great) and wonder which method will give better results. Also any opinions on which places give better service?

 

[Platypus]

Call around and find out. Im sure they could tell you. I know kinkos does blowups.