The Acer XF240H is a better monitor over-all, but not to those unaware of the 144hz AUO TN panel wash out, or loss of contrast, gamma and uniformity 144hz 1080p AUO TN panels suffer from when set to 144hz versus 60hz. The XG2402 is less dull or washed out at 144hz due to the higher preset gamma, but the skewed gamma curve causes some light colors and shades to be too dark and dull, and it suffers from an obvious blue tint to dark greys which can only be fixed by an ICC profile created by a colorimeter and quality calibration software. The XG2402 also suffers from obvious color streaking and a bit of smearing out of the box since its best overdrive (Rampage Response) setting is not the default setting.
Both monitors can be significantly improved by changing two menu settings which affect the gamma; once their settings are changed the Acer XF240H is easily the superior monitor for 144hz gaming, but it lacks the multiple Custom Menu settings the XG2402 has which allows one to save three sets of settings, such as one setting for 60hz (console use) and one set of settings for 144hz since the different refresh rates need to have different settings to ensure the best accuracy at the most important refresh rates (60 & 144hz). The Viewsonic XG2402 also offers slightly better calibrated image quality when an ICC profile is active since it can fully cover the HDTV/REC 709 and sRGB color space, but this is irrelevant to the majority who do not own a colorimeter.
The Acer XF240H has higher contrast and better uniformity, as well as did not have a stuck red pixel like the XG2402 I tested, but these issues could be the result of the panel lottery being in the favor of the XF240H I tested. Lastly, the Acer XF240H has a bright matte grey bezel which vastly increases the perceived black depth and helps high the contrast and uniformity loss. The XG2402 has a matte black bezel which reduces the perceived black depth, and is an odd feature to change since the XG2401 had a bright matte grey bezel. Viewsonic could have saved money by continuing to use the XG2402's predecessors casing, and instead invested the money and time into improving the preset color accuracy and overdrive/pixel response time performance.
I prefer the Acer to the Viewsonic, but without seeing more reviews it's hard praise it since I don't know how consistent or non-consistent it is image quality wise. All of the 144hz AUO TN panels suffer from some degree of wash out at 144hz, and some are clearly better than others. If similarly priced the LG 27GM79 (set gamma to 4 at 144hz) is a better value than both the Acer and Viewsonic since the LG has both AMD Free-Sync and back-light strobing. If after better image quality both the AOC C27G1 and Samsung C24GF73 are great choices, but they suffer from horizontal gamma shift, center crush and have less sharp text than the TN panels. I'd also recommend the Viewsonic XG2401 over the XG2402, but not the XF240HA assuming my unit represents the typical performance of an XF240HA and was not a panel lottery winner.