GPU mining income is way more stable. RX470 were making ~$1 per day a year ago, still making $1.5 per day now. As for ASICs, just go to forum threads about Antminer D3 and you will see how happy people were in the beginning making $100 day and how pissed off, getting $5 per day with no chance of ROI. Much shorter warranty on ASICs is another factor to consider.