Your summary of H8 is just about right. H8 is newer, more powerful, and has a lot of development work going into it right now to get the firmware right.
H8 is taking a different route in Kodi to Q5. Q5 loads external player (via wrapper) whilst Himedia (and others) are developing custom Kodi specifically for H8. The advantage of the H8 approach is that it will work better with add-ons and Kodi menus. Eventually this Kodi work should be rolled into standard Kodi so future Kodi will have inbuilt good support for H8.
playback of music and 1080p video from file or iso from a samba share? No difference between the two. Kodi probably won't even need wrapper for 1080p content in Q5 and H8 Kodi will handle this easily. Differences occur with high bitrate files, 7.1 audio and 3D. All of which Q5 with wrapper is currently better, but probably won't be in a few months time. Samba shares cannot be password protected (there is a way around this but it's complicated), this is an Android limitation.
-can I output optical and analog audio simultaneously? what about HDMI? (all three) No. Output is via one audio output at a time for both H8 and Q5.
-noise? is it fanless/silent? how about the heat? Neither have a fan and both are silent. Q5 runs quite warm but we've been selling it for a year and it hasn't caused problems. The casing is metal so is designed to get hot / dissipate heat. H8 runs quite cool regardless of what you throw at it. Again casing is metal, which is best for dealing with any heat.
-remote.... does it map intuitively to the Kodi UI? (is it wife proof?) Yes. In both cases. Everything works as expected with the one exception of the remote home button exiting Kodi and returning to Android home not Kodi home. You do get used to this.! If you're mostly interested in Kodi then I'd recommend setting it to auto-boot to Kodi (instructions in forum FAQ), this will have the nice side-effect of auto re-loading Kodi if you do accidentally hit the home button. Auto loading to Kodi will also make it significantly more wife-proof!
-other controls... Could I plug in a USB game controller and use it like the old XBMC mapping? (always found this the best for cuing up music playlists on the fly) Yes for both Q5 and H8, but it's not straightforward. See here: http://www.futeko.com/newforum/index.php?topic=322.msg2232#msg2232. Not sure about XBMC mapping. If you have a smartphone I'd recommend a Kodi remote app. Or for Q5 there's anAndroid app called Hicontrol which mirrors Q5 output and gives touch output on phone/tablet. It's very good. At the moment it doesn't support H8 but should do in future.
-add ons... Havent done much in this area in the past but know there are many. Are there any issues with loading kodi add ons, or android apps? I haven't found any issues on either box, but read the forum for more info. In some cases people seem have a problem with H8 where add-ons don't appear, but we can't replicate it and it's fixed by a reinstall. Most add-ons work well in Android Kodi after version 14.2. Earlier versions had problems but with Android Kodi being very popular the problems have been fixed by Kodi devs. All Android apps should run on both boxes. Some might require a mouse. Some streaming apps may block content based on IP or other methods.