I have been getting my 12x12 layouts printed at Costco's 1 hour printing. A friend recommended Scrapping Simply so I decided to give them a try. I just got my first order. Here's the breakdown:
They charge $1.99 per 12x12 print and shipping is a flat $5. (for the US. They ship international too.) Costco charges $2.99 (plus tax, but no shipping) so as long as you order more than 5 prints, scrappingsimply.com is cheaper.
I uploaded layouts and placed my order on Monday morning. Late Monday afternoon I got the shipping notice. The prints arrived Friday. That's from Utah (I think?) to Maryland. I suspect they didn't actually get to the post office until Tuesday. Not a bad turn around time, all things considered. Obviously 1-hour printing at Costco is faster, but you do have to go pick that up. ;-)
They both print on the same photo paper - Fuji Crystal Archive paper. They both offer glossy or matte (lustre). Costco offers the ability to turn color correction on or off. I find turning it OFF for layouts to be essential. It may be that scrappingsimply.com doesn't do color correction, I don't know.
I'll have to do a more thorough comparison of the print vs. file on my computer but my first impression is that scrappingsimply.com trims less than Costco. It may be an illusion of the layouts I had printed, but I could tell right away that my Costco prints were trimmed. Wasn't as obvious with the scrappingsimply.com ones.
The layouts were shipped in a sealed plastic package inside a cardboard box. No problems with bent corners or wrinkled layouts.
Overall I think the two companies are equal quality-wise and I would recommend either. Obviously if I continue to use scrappingsimply.com I will wait until I have large batches to print to make the most of the $5 shipping. Often I am too impatient to wait until I have more than a few layouts done to print them. :-)