What would you trust first?

My refractometer reads 0 with water. My hydrometer reads 0 with water. My refractometer says my OG is 1054, but my hydrometer insists its 1050. I was aiming for 1056 on a system in know pretty well, so I'm inclined to lean towards the refractometer reading.

Does your refractometer have a brix scale, and does that agree with your hydrometer? It's possible the conversion is wrong. I read on here that there is a common I've online which has a bad conversion.
Otherwise, you have one data point, the way to tell is to dissolve a known quantity of sugar in the correct amount of water, then test both. 1 brix = 1 gram sucrose in 100 grams water. Brix to sg conversion is easy. Google should turn up the factor. I think 10 brix = 1.040. Might be wrong.

hydro

Definitely hydrometer. Your refract is designed to measure the concentration of sucrose in water, not wort. Many refract's have a scale that attempts to account for the average difference in refractive index between sucrose and wort, but its just an estimate.

If you want an exact number, trust the hydrometer. If all you care about is being close enough a refractometer with correction factors is adequate.