I have some other rigs running but I am looking to building another rig so help me to choose the best setup for mining, 2x RX5500XT or a single RX5700XT? Which one will be better in return of investment wise, energy consumption and cost
It depends on your your conditions and capabilities. RX5700 better if you have not enough free space. It makes rigs more powerful and compact but have problems with cooling and flashing BIOS. RX5500 are better in cooling and reselling. Right now the price per hash for all GPUs is mostly the same. Just buy anything you can find.
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
My RX5500 gives 28MH mining ETH without problems.
That's true, I find RX5500xt to be cooler than RX5700XT and the 5500Xt is more power efficiency I guess that's why the cooler works better, combining two will give rest of mind than RX5700xt and if RX5700xt is a must one must get the three fan variants