They are two different guns. The SL8 is meant to be somewhat of a precision rifle with a heavy 20" barrel, thumbhole stock, and takes proprietary single stack mags. IF you're looking for EBR styling with a pistol grip, folding stock, flash hider, and hi-cap mags, then it's not for you. It's expensive to convert the SL8 to that configuration, both in conversion parts, and 922r compliance parts. For the SIG 556 vs. 551-A1, the only choice is whether you want to use cheaper and easily available AR mags or the more expensive and harder to find Swiss SIG mags. Other than a few minor cosmetic differences (color and flash hider), they are the same gun.
However, quality is not an issue with the HK. I have a converted SL8, plus both SIGs, and there is nothing wrong with the HK quality. Polymer guns and parts have been around for quite a while, and they are not inferior in any way when properly done. The SL8 isn't weak or easily breakable, and if you look closely at the hammer, it has a steel striking face embedded in the polymer. "Plastic" G36s have been used all over the worlds by military and LE, and are the standard arm of the German military. Lots of "plastic guns around, from the AUG to the FN P90 and F2000, and other guns have a lot of polymer like the SCAR and ACR, so I think people have gotten the hang of using polymer in guns.
I have a number of HK and an number of SIGs, and I'd take an HK over a SIG any day for quality and reliability. Note I'm speaking of the US made SIGs. The original German made SIGs are a different animal.