I'd suggest you get some sample prints from whatever service bureau, on whatever printer, on whatever paper/canvas you are considering. Sample prints of a detailed section of one of your own prints. That way, you can answer your own questions with your own images about quality, etc.
Crop out a section of one of your prints at the same size as the paper/canvas that you'll order for the sample print. For example, (in english units) after sizing your finished image to 50in X 60in at 300dpi, crop out a section at, say, 11"x14" with detail that you are interested in. Have that crop printed at 11"x14"by all printers on all paper/canvas choices that you are interested in. See for yourself what your image will look like prior to placing a huge, important order that you might not be happy with.
Finally, prior to sending the file for printing, be sure that you softproof your sample images and final images using the service bureau's profiles for the printer/paper(canvas) that you're considering. You'll almost certainly want to adjust them prior to sending for printing.
Good luck
Brad