Both won Gold Gloves in their career but that often doesn’t tell the full story. Walker played much of his career before the invention of defensive metrics that many teams use to evaluate players today, like DRS, or Defensive Runs Saved, or UZR, or Ultimate Zone Rating. Prior to 2002, neither one of these existed and Walker only played through 2005. However, prior to that, the most accurate stat that was used to evaluate defense was TZ, or Total Zone. Like DRS and UZR, above zero is above league average and below zero is below league average.
For Walker, he won seven Gold Gloves and in right field in his career, and from 1989 through 2001, Walker had a TZ of 86 in right field, according to Fangraphs. He also had 2 DRS from 2002 through 2005 but he had zero in 2002 and 10 DRS in 2003 so you can safely say that his fielding deteriorated in his final two seasons.
For Helton, from 1997 through 2001, he had 33 TZ at first base and from 2002 through 2013, he had 29 DRS, according to Fangraphs. For these defensive numbers, there’s no exact science to them and especially with TZ, it can be a bit spotty but it can give you a rough estimate. Heck, even with the DRS, Fangraphs has 29 DRS and Helton’s Baseball-Reference page says that he had 32 DRS.
However, based on the combined numbers of 88 for Walker and 62-65 for Helton and Walker winning four more Gold Gloves, Walker is the winner here.