it's depending about the question, for short time i will choose altcoin like ethereum, it's more faster ROI than mining bitcoin.

That is right. The Etherem is good to mine at present. But the mining difficulty is rising too fast, so the profitability is dropping all the time.

I have some old generation cards I bought 3 years ago. It seems their mining hashing is dropping all the time.

If you have the AMD R89 380 or later cards, such as R9 390, their hashing rate drops much slower. But they are expensive.

Do you know anybody tested the latest AMD RX 480 or 470 with different Dag file size. Maybe it is not necessary.