First and foremost, these two have been offered by a freind, and having purchased the 28, am allowed to upgrade to the 800 if I wish. This pal has gone Canon.
Oddly, although a technician, he recalled the TTL working on his D70. But the forums support your statement completely. I guess he wasn't really into it at that stage.
The 400 sounds more practical for my needs, but just isn't in the offing.
I have a large flash from year dot. It works well, but someone on this forum I think, suggested that there might be large voltages across the trigger circuit. There were: 200v ! I never risked it again. The adapter cost $25
I do feel the little on-board flash is inadequate. I was at a large party here in Austin, and I just was not getting that zest to the picture that I expected. The 'new' flash really fills in the contrasty areas, or floods the dark corners if you want. It's throttled back for most of the range, for most of the time, so doesn't use much of its own power and saves a lot of camera battery life while it's at it of course.
One problem is that I don't have a manual. I'm searching the net, but so far am getting caught up in blog-like over complexity. My pal has the manual to the 800 and may find this one.
One clear statement on a forum was that you have a lot to learn - so take the manual even when you think you're up to speed on the device. The way my memory is, I'll have it mounted on me specs.