have never liked prints from snapfish personally – and there are big threads on flickr complaining about the quality of their prints now flickr have partnered with them.
photobox good, but come out a bit dark for my taste, despite having a properly calibrated monitor and sending sRGB files to them
you might want to take a look at DS Colour labs in Manchester (www.dscolourlabs.co.uk) – they do nice normal prints and are pretty competative on canvases (cheaper than photobox) – have never used them for these but expect them to be ok and have heared good things re their canvases.
Personally I like Loxely Colour – pricey, but worth it, but you do need a trade account with them as they dont sell to the general public