The state of play late 2014.....
First rule - don't even look at an Android based box if you want perfectly 100% synced video, with good quality XBMC Live TV de-interlacing.
This excludes the Amazon Fire TV.
These boxes are unable to switch display refresh rates on the fly to match up with 25Hz, 50Hz and 23.076fps (24p) video.
This may change when Android 5.0 Lollipop is more widely distributed.
XBMC use only with perfect switchable on the fly video + good support here on the forum: (very very important!)
1. Cheapest - Raspberry Pi B $35 (case + power supply needed, WIFi dongle RT5370 - if required)
2. Medium - ASUS or HP Chromebox $140 ish
3. Most - Intel NUC with Haswell Chipset and HD 5000 Graphics $350 (bare bones - expensive once parts added)
4. Expensive / Integrated - late 2014 Mac Mini - Intel Haswell equipped as well. $499
The top three will all need some sort or IR connector and a remote control. The IR is integrated in the Mac Mini, no remote however - a Harmony remote works well with this once programmed.
One distinct advantage with the Raspberry Pi is HDMI CEC control - you can control XBMC basic functions with some TV remotes straight out of the box.
If I had my time again and wanted to future proof the system down the track somewhat. I would get a
Chromebox, Flirc IR dongle and possibly a quality programmable Harmony Remote.
The Flirc can be programmed to learn any spare remote you have lying around the house to control XBMC.
Run the XBMC version - OpenELEC on it and it will literally fly.
Chromebox and 4K TV:
http://forum.kodi.tv/showthread.php?tid=202201
This gets you an easy to setup system, with a great range, quality and
flexible IR remote system that can be programmed with shortcuts for XBMC and learn IR commands for your TV as well if you get a Harmony.
Then just get a cheap Chromecast HDMI dongle if you need Netflix etc...
http://kodi.wiki/view/Chromebox
https://flirc.tv/
http://openelec.tv/
http://myharmony.com/