Its closer than you think.
having owned both sets of cards for a decent length of time I can say with confidence that if AMD have worked with the developer to optimise a game, the results are generally far better than the Nvidia gameworks rubbish they have been producing in the past 6 months.
You can really see a stark contrast in performance where AMD and the Developer have collaborated and produced great results. They don't have many games like this but when they strike gold, its a home run i.e. really great frame-times and smooth perf. beyond what you see in most games, whether its running nvidia or amd.
I also find the ability to enable 1,2,3,4 GPU's in increments very useful on AMD, on my Nvidia setup I need to enable a single card or all 4 at the same time.
I have found the extra software Nvidia bring to the table very good, I really like shadowplay.
AMD's multi-monitor support is superior and I have some annoyances with the Nvidia driver in terms of the way it handles the switching between primary and secondary monitors. It has a bug which I need to work around every-time.
That said, Nvidia have been first to the table with launch-day game ready drivers in some scenarios and for some reason, my favourite game, Elite: Dangerous is lacking AMD Crossfire support whereas SLI is supported.
I was stung badly with the 7970 driver transition, it took just up until around the announcement of the 290X for things to sort themselves out. I was running for 9 months on the original pre-7970-release beta driver. In the end they fixed pretty much everything but by then it was too late, the PR damage to AMD had been done.
I did not switch away from AMD because of the drivers, they were very tight when I switched over the 980's, I just could not run the 7970s in my rig any longer, they were too hot. Now I have switched, my angst is more against the Game Devs than Nvidia, the devs have been churning out some half arsed junk lately in terms of 3D engine code. Some notable bright spots aside (Code Masters, Crystal Dynamics, MGS TFP, Unreal Tournament 2015).