I'm not going to repeat what others have said here, but the one option that neither 3speak or DTube seem to have (correct me if I'm wrong) is the ability to embed video on other platforms in the way you can with a youtube video. If I copy/paste a link to a youtube video in a Steem post, the youtube video can be played on my post instead of redirecting someone to another website.
While this might seem great to get people visiting 3speak, it's an irritation for the user who wants a seamless experience.
To be fair to 3speak, I am not certain how much the built-in features of a web browser play into enabling embedded videos, but if there is one feature that needs working on, it's the ability to watch a video from steemit.com or steempeak.com, rather than being redirected to another website. I want to watch a video from a steem post, not go to 3speak.
Just my opinion, but I believe between DTube and 3speak, the one to succeed long term will facilitate (in whatever way possible) the ability to watch videos from within a post. Even this post and your video redirect me to 3speak website, which doesn't make the experience seamless.
Like I said in a previous comment. This already works on steempeak, just not on steemit.com. If a website wants to embed 3Speak websites they are free to do so by using the already existing embeddable video player.
check it here: https://steempeak.com/@exyle
I'm reading this on steempeak and clicking video directs me to 3speak.com
All the videos in 3speak could be ember in twitter just by sharing the link. You can embed them in your website just by clicking under the video the button share and use that link. And they could be ember in forums if the forum accept the code. From steempeak, I'm able to watch and I think steemit will add that option in the future.