A designer would always choose Helvetica over Arial, but a lot of people confuse the two. Yes, if you look at them quickly you could mistake them but with a little help they are pretty easy to tell apart. A little background on the the faces.
Helvetica was designed in 1957 by Max Miedinger, Helvetica’s design is based on that of Akzidenz Grotesk (1896), and classified as a Grotesque or Transitional sans serif face. Originally it was called Neue Haas Grotesque; in 1960 it was revised and renamed Helvetica (Latin for Switzerland “Swiss”).
Arial was designed in 1982 by Robin Nicholas and Patricia Saunders for Monotype (not Microsoft), it’s classified as Neo Grotesque, was originally called Sonoran San Serif, and was designed for IBM’s bitmap font laser printers. It was first supplied with Windows 3.1 (1992) and was one of the core fonts in all subsequent versions of Windows until Vista, when to all intents and purposes, it was replaced with Calibri.
Below you can find some resources on how to spot the difference between the two faces. If you think you’re good enough to tell the two apart you can take some comparison tests.