At the time I got mine, around summer 2014:
1. 2560x1080 is much easier on the GPU, to achieve a consistent 50+ FPS in the latest games @ 3440x1440 without completely dropping settings was and still is very tough, you ideally need a top end GPU i.e. a 1070 or preferably 1080 for some of the latest titles, not as much of an issue with fre/g sync models floating around now but even with that tech. I still wouldn't want to be dropping below 50fps
2. far cheaper, £300 vs £900, imo the extra is/was not worth it
3. the 29um65 did a bit better than the LG 34" 1440 model for input lag and response time
EDIT:
Also, the 29" models seemed to have better contrast ratio of about 1200/1300:1 where as the 34" wasn't any higher than 1000:1