M290x vs M380? How much of a step down?

Edit: Should be m390 in the title.

I just purchased the late '14 27" refurb iMac a week ago. It's got the m290X. I'm still within my return window.

Now the new mid-tier 27" would end up being only $50 more than what I paid for the '14. It'd be an easy return and re-purchase.

But how much a step down is the M390 in the new iMac from the M290X? I only game a little, but I do play on occasion. Are the other improvements enough to offset the drop in graphics performance?