I think both events would be similar in the force applied at the start. The 400h with a little more rhythm involved may be just a tad bit more controlled.
There is a reason they are world class 400h and not 400 runners.
The very best 400h obviously have to be able to run fast over 400 metes, but they are not going to be in the top 15 in the world. My belief is that what is gained in hurdle rhythm holds back gains in speed to run a fast open 400.