I use H&H and am generally happy with their products. I also occasionally use WHCC, and generally the quality seems very similar between the two.
I'm not a fan of the EZSuite ordering software for H&H, but with some practice it's getting a little easier to manage. It got a LOT easier once I realized that H&H provides templates for almost all their products, showing the expected bleed/trim areas.
I wasn't particularly happy with the one press-printed book that I ordered from H&H, but their photo books & albums have been very nice. Press-printed cards have been fine, and very similar in quality to WHCC press-printed cards.
I REALLY love H&H's prints on metal. If you get the "alluminescence" metal print, you may need to boost your mid-tones, depending on how you expect to display the piece; the aluminum isn't white like paper and certainly not bright like a screen - but if it's displayed under a light then it really pops and shines. Do some small metal test prints before committing to big metal prints.
I haven't ever ordered metal prints from WHCC.
I have not ever used H&H for canvas prints.
H&H packaging is great, perhaps even overkill.