You can see how much the difficulty has risen over the past month or so
here.
Look at the last month and its gone from 370 to 656 (as of the time of this writing) thats a change of 286.
One of my rigs which gets around 171 MH/s generates about .2 ETH every 38-45 hours give or take. At the current price of $279 is about $55.80 for every .2 ETH I'm generating now minus power, this also doesn't include Decred which is rising in price and helps pay any power.
When ETH changes to POS and makes mining for ETC obsolete you can always go to other coins. I believe we will see the price of ETH sink, as to how far who knows.
You also have the option to mine ETC which current
difficulty on another rig doing around my same hash rate as my ETH (171) I'm pulling in 1 - 2 coins a dayish. However ETC has been stuck at the price of $17 for some time now and who knows if we will see it go up. Difficulty change in the last month for ETC has only increased by 15.
I'm sure we can do the math if you mine ETC, lets say 1.5 coins a day running at around 170MH/s at the price of $17 would be around $765 plus any bonus money from Decred after power is paid, which then you could by some ETH if you wanted to with it, or hope that ETC's adoption and price goes up.
Its all a game you need to figure out yourself.
However if your thinking ETH difficulty will go down, I have to say its only going to get higher as people can get their hands on more GPU's (prob another big wave of them in July) until you cant mine ETH anymore due to switch to POS.
Enjoy it while it lasts!