Both monitors share the same specifications, with 24.5-inch TN-panels with 3-sided narrow bezels, 1920x1080 resolution, 240Hz native refresh rate, 400 nits of brightness, 1000:1 contrast ratio and 1ms response time.
Both are likely to feature four USB 3.0 port hub, headphone jack as well as DisplayPort 1.2 and HDMI 1.4 inputs. According to what we can see from the product page, both also have Alienware's fully customizable RGB lighting system on the back, including the small alien head logo.
This is the perfect example showing how much cheaper it is to make a monitor that supports FreeSync as opposed to the Nvidia G-Sync, as there is a US $200 difference in price between these two models.
The AW2518H, with Nvidia G-Sync support, will sell for US $699.99, while the AW2518HF, with AMD FreeSync support, will sell for US $499.99. Both are expected to be available in the US today.