Ya if they rx560 cost the same as the rx550 i think i also would buy the rx560.
But the rx550 does hash at same speed as rx560 with lower power consumption ( about 35 vs 22 watts in GPU-Z) This is for cryptonight algo only.
But the rx560 is faster in ethash and equihash, so it really depends on what you want,  where i live power costs 24 cents/kw/h, so for me the rx550 is better buy.
If you only pay 5-10 cents for power i would buy the rx560.