comparing prices/quality of a costco and mpix or WHCC (which is the lab is use) is like comparing a ugo to a sports car there is just no comparing the prints.
Costco may do prints cheap but at the expense of quality, there whole goal is high volume which doesnt mean high quality in a lot of cases unless you have a highly trained operator who knows about color management.
Costco maybe great if you need them quick but for the most part i send out my work in enough time to allow for the turnaround time of WHCC (White House Custom Color) as if i get the order uploaded by 9am monday i will have them on wednesday afternoon. I can get prints of any size i would like from 4x6 to prints from 30 in tall by how ever long I need them. The quality is top notch and i havent had any problems to date and its on professional.
The biggest problem is that most labs dont have a RIP (raster image processor) installed on there machines so the file just gets blown up and doesnt look all that good because the detail isnt there. If the lab has a good rip or a copy of genuine fractals or similar programs they can do a good job of it but the quality of the orginal file has to be good to start if not you only enlarging a bad shot.
A good base for judging how far a digital file can be enlarged before needing other methods then a rip is take the file and make the DPI settings to 150-180 DPI for RA-4 prints and 200-240 DPI for inkjet and see how big a print it says it can go and then double it thats about as far as you want to go without processing the file differently, viewing distance also plays into this as well. Then in my prints im much more discerning on the quality of prints so what may be acceptable to the average person isnt proof quality to me, but then Im not the average consumer.
i will second the Kodak endura metallic paper as it can make a image pop off the paper and appear almost 3D. As i have a custom print edition that i just released that is on Metallic and the image pops off the paper and no image i could post online would do the actual print justice.
As for the portraits I have done portraits on them (Color and BW), as one of my best portraits in my portfolio is on metallic as the models eye color(deep saphire blue) and clothing(purple corset) just screamed to be printed on the paper.
With metallic you have to be very careful on the skin tones in your processing as it has a tendency to saturate the blues a little and lower the contrast in the reds. With proper color management and profiles i dont have a problem.
