R7 240 4GB = 984 H/s
You're a little low, may want to work on settings - try a lower LG or increase frequencies. You're leaving hashpower on the table...
R7 240 4GB = 1.18 KH/sec
So 240 4gb and 750ti are 2 most efficient cards to mine yac right now?
I just want to try the coin especially with much greener setup.
From a price/performance ratio, yes.
The R7 240 4GB can be had for ~$70-$80 and draws 25W under YAC mining load
The 750 Ti is $130-$140 and I understand is around 35W under YAC mining load
If you
just want to mine YAC, go the R7 route. If you want a card that is good for a lot of things that happens to be decent mining YAC, spend more for the 750 Ti.