Both have advantages over one another that I wish the other had.
The Avengers nailed the team dynamic. From the patient build up of the first act, where we are reintroduced to each of the disparate Avenger that will later come together. Whedon is clever to infuse every one of those scenes with the psychological tacks that makes these characters tick. And then he holds that logic throughout the course of the film as these characters, these personalities, these egos interact, clash, explode like an inevitable time bomb and reconcile with each other in completely organic fashion. Whedon gives well coordinated screentime to everyone to matter. Especially and thankfully, Cap. During the entire Hellicarrier siege, Stark gives Cap the orders. It feels wrong. I was actually scared that it would be more of the same come battle time. But no. Whedon simply used the upper hand Stark gained (to fruitless results) to later make the fact that Iron Man and the rest taking orders from Cap more rewarding. And Avengers take the marching orders as part of a perfect orchestra, each having a beautiful part to play. As good as X-men was, it was once again the story of two or three central characters. The team dynamics muted to the framing introductions and conclusions of the picture where cyphers with two lines or less compete for our apathy.
But whereas you care about the characters, you find it hard to care for their care for the safeguarding of NY. The Chitaury are scary, then you see them move. You see them easily be dispatched by mortals like Widow and Hawkeye. You wonder why you should care? Their boss, Loki, is also ineffective here. Without the emotional betrayal and insecurity driving the plot as in Thor, Loki's villainy lacks tangible conviction. The film knows this. The film, through Coulson, points this out. Loki is an ineffective villain. But the film being self aware of this fact does not change the fact that the climax is totally tensionless. The Avengers have a breezy ride once they assemble. This is where days of the future Past positively radiates. The fact that you are mortally anxious for the X-men's (the core ones, not the ciphers) well being despite the fact that time travel makes death and damage naught is a testament to the Sentienls. They were truly formidable. More importantly, Singer makes the stakes viscerally impacting. When Nixon is on the verge of death, you cringe, when Mystique is toying with her trigger finger, ready to fire, you plead her not to. In an age where even "smart" comic movies like Cap TWS, BB, and TDK must coalesce with all out mayhem, Singer displays magnificent restraint. He eschews traditional spectacle and focuses on the seminal choices of the characters. The destruction is only there in the background, to serve, not be served. Yes, his team work needs work. But damn it if you don't care for the characters that do stand out from the pack. After seeing it twice and having weeks to ruminate on this, Days of The Future Past may just have the finest third act in a comic film ever.
If only the two aspects both of these great films were combined somehow. We would get not only the ultimate comic book film, but the most supreme blockbuster.