Worldwide, I think Infinity War has it in the bag, but I don't think it will win domestically.
Age of Ultron managed 1.4B, and IW has a lot more hype and good will going into it. Black Panther managed 675M as a cultural phenomenon and did so without any real competition. IW will have both Deadpool and Star Wars to deal with a mere month after its release (not to mention the tail end of BP itself). Even if it is a cultural phenomenon itself and better received than Avengers 1, it will be facing a lot more opposition.
600M is an insanely high bar that A1 (a phenomenon of its own) barely managed to top. Hardly anything beats it even under the best conditions. IW has the potential, but that 700M mark (which BP should get pretty close to) may simply be too high for any movie to achieve when there is strong competition early in its release cycle. IW will have to steamroll another superhero movie and another all-ages spectacle just to hit 650M.
I'm just not convinced that it has the domestic box office in the bag, but if ANYTHING can pull it off, it's Infinity War.