A single RX5700XT will do 54MH per second on Ethash algorithm and a single RX5500XT will do 26MH per second, so 2x RX5500XT will give 52MH combined, now 52MH vs 54MH per second, the difference isn't that much, also the power draw isn't that much too, where the real deal come from is how much you will be getting the cards, go for any that's cheaper