Jeri Ryan stated in an interview for the September 1999 issue of Playboy magazine (no, she does not appear nude) that she asked the producers of Voyager if she should pronounce it "few-tile" like other Borg, but she was explicitly instructed to say "few-tul":

PLAYBOY: Speaking of the Borg, is resistance "few-tile" or "few-tul"?

RYAN: Good question. When my character was introduced, I had to say, "Resistance is futile." The producers had shown me the movie First Contact so I'd at least know what a Borg is, and every time a Borg speaks the line it's "Resistance is few-tile." Few-tile, few-tile, few-tile. So I asked your question: "Few-tile? Is that a Borg thing? Or is it few-tul?" They said, "No, no, no, it's few-tul. You don't say few-tile. Patrick Stewart says few-tile because he has an English accent." I said, "Well, what about the voice of the Collective? It says few-tile, doesn't it?" "No, no, no. We recorded the voice of the Collective and it says few-tul." I said, "All right, but I don't want to take the flak if we start getting mail because I said the wrong thing." Sure enough, the show airs and the voice of the Collective says few-tile, and I'm the only Borg in the history of Star Trek, apparently, who has ever said few-tul. It has no zip. It's depressing.

The website Ex Astris Scientia notes that the only other instance of a character pronouncing it "few-tul" is Data in First Contact, ironically the movie that Jeri Ryan cited in the interview.

Actually, Data says "few-tul" too in "Star Trek: First Contact" (when he smashes the coolant tube, thereby killing the Borg). Which appears strange because Jeri Ryan should have noticed that, saying that she watched the movie. Perhaps she only saw the introduction of the Borg in the beginning. And after all, she explicitly referred to the Borg always saying "few-tile", not to Data.

It's also possible that Jeri Ryan did in fact notice that Data said it differently, which prompted her to ask the producers "Is that a Borg thing?" Or she just forgot.

In-universe, both Locutus and Seven were assimilated humans. Its not unreasonable that they may have retained small details of their native accents that are below the level of resolution considered significant by the Collective.

This is an issue of localization.

The Borg had assimilated hundreds or even thousands of species at that point, and none of those species spoke English except for humans. (And actually, the Federation being such a tolerant culture, it can also be assumed that many humans still speak their own native tongues.) So the Borg can be assumed to already know thousands of languages. Probably their policy is to issue their message in whatever mode of communication they think most effective at the moment.

After assimilation, it would appear that individual drones don't talk to each other much. So the main reason Borg talk at all is to announce their intentions to new assimilation targets. The Borg are not only incredibly determined, but also highly flexible, in their own way; they wouldn't sweat too much over learning a hundred different languages in order to communicate, much less two mere dialects.

Standardization is certainly a core Borg value, but they still recognize the need for variation in some contexts (spheres vs. cubes, for example). To draw a real-world analogy, many modern software packages distinguish between British English and American English--why wouldn't the Borg?