IOU is better than vIOS, because it's very lightweight and requires much less resources.
IOU is better than Dynamips simply because Dynamips is outdated and cannot run IOS 15.x (with exception of early 15s for 7200 platform which still lack a lot of features)
Regarding EVE-NG vs GNS3 flame war - it's really a matter of taste. I run both and see only one advantage of EVE - it's clientless. But that advantage is HUGE.