Three months ago I replaced my 24" 1920x1200 Samsung with a 27" 1920x1080 Asus screen. I am still not decided if it was worth the money.
My main reason was to get lower responsetimes. My older Samsung 2443BW 24" has 5ms response time. I always found the screen blurry when I moved the camera. So I wanted a monitor with only 2ms. I chose the Asus VE278Q 27". It's impossible to see different monitors in shops these days. And reviews of monitors always seem very subjective. So I had no other choice but to just try.
Motion in games is indeed a little less blurry. But not by much. Having 16x9 in stead of 16x10 is slightly less enjoyable on the desktop. Maybe also in MMOs, where you have a lot of UI stuff at the bottom. But in other games, 16x9 is just as good as 16x10. There are 2 other benefits to my new screen (besides slightly less blurry). One is the size. 27" is nicer than 24". But I also realize, I don't need anything bigger than 27". I don't think I want a 30" monitor.
The second benefit is the higher fps. 11% less pixels indeed means 11% more performance ! At the time I was playing mostly Guild Wars 2 and Skyrim. All settings to Ultra. Enabled SSAO on the control panel. Skyrim visuals were heavily modded. Both games ran 60 fps in most places, but some spots had only 50 fps or less. (gtx680 + 3570k@4Ghz). I wrote down numbers in some locations in both games. And after switching monitors (and resolutions), to my surprise I saw a ~10% fps increase in almost all locations ! In theory 10% less pixels means 10% higher framerates. But that doesn't mean you will see that result in practice too.
For you, the higher resolution also has the larger screen. That is another factor to consider. Do you want a bigger desktop at work, or bigger screen while gaming ? But I would certainly look at response-times. Your Samsung has 5ms response-time. Not sure about the Dell (I see 2 24" 1080p Dells, with 5ms and 6ms delay). But if both have 5ms delay, and you find 10% higher fps useful, I'd certainly swap the monitors.