There is no way anyone can ever tell you the ROI / profits you are likely to see. Anyone who tells you they can, is a liar.
I use profit calculating websites like whattomine.com and
https://www.coinwarz.com/cryptocurrency and
https://www.nicehash.com/profitability-calculatorThose websites can give you a rough idea of what you are likely to earn AT THAT VERY MOMENT. However, the next day, it can completely change. And it might be really profitable for months so you keep all your coins, and then the price plummets and your coins aren't worth much anymore, equally, you could sell them regularly and then the price sky rockets. It's all about personal preference and risk assessment, just like a normal stock market.
The coin you mine would depend on what cards you buy. Ethereum is a strong currency (second only to Bitcoin) with a big following etc. but that makes it difficult to earn many coins. A lot of people mine smaller coins and trade them for ethereum etc.
Personally, I am mining ZCash because I have older 280X cards that aren't very good at mining Ethereum, but my new rigs will likely mine Ethereum.
Totally understood, profitability is the biggest concern obviously since I dont know how long this sweet deal will last. I can trade the currencies myself on the exchange. I guessa better question would be, why switch to Ethereum if you could generate more Zcash coins with your new rig? Just for the risk of zcash dropping off?