The cornerstone to original letterpress artwork is having plates made. Not that I've worked with anyone else to have my plates made for letterpress, but I love working with Owosso Graphic Arts. I don't know how they compare to photopolymer plates since no one I know uses both metal and photopolymer plates, but I'm sure that there are advantages and disadvantages to either type of plate. In addition to having lightning fast turnaround (same day for magnesium plates if you get it in by 10am Eastern), I just learned two great things about ordering from Owosso Graphic Arts.
1. On a two color vector image, you can leave the entire file intact and they will be able to separate out the two colors and make a different die for each color.
2. This I learned from one of my classmates in my letterpress class. You can also save money on plates by laying out all the different plates you want made into a single file and submitting it as a single plate that needs to be cut into different pieces (the cut charge on wood-mounted magnesium is 60 cents per cut). I'm going to do a more detailed post on this later when I have more time. Basically, I saved about 40% by submitting four plates as one plate that needed to be cut instead of submitting them as four different plates.
Call Owosso directly (800-444-5552) if you have any questions about the best way to submit your artwork. They are super helpful and responsive, especially over the phone.