Below, I am going to outline what to expect from various groups of devices given the type of OS that is installed on each device. Note with any of these devices, it comes down to having the correct hardware (regardless of what they look like on the outside) - Having the correct software, including drivers, kernel, and programs – and having everything setup properly to enable smooth playback.
Arm Single Core (AMLOGIC Devices):
LINUX & ANDROID
Single Core Linux XBMC boxes are still very fast compared to their Android counterparts. With a upcoming cut down version of Android, XBMC can still run pretty quick to stream 720p videos.
Arm Dual Core (AMLOGIC & RK3066 Devices):
Dual Core Linux XBMC boxes (AMLOGIC ONLY) are going to be the fastest lightweight, inexpensive XBMC boxes. They are still under development though. With any luck they will be ready soon!
Unfortunately, the RK3066 devices do not support hardware acceleration under the linux OS and therefore cannon playback videos very well under XBMC.
Dual Core Android XBMC boxes are just fast enough to run Android and XBMC. The interface lags a bit and videos can take an extra couple of seconds to catch up when starting or fast forwarding.
Arm Quad Core (RK3188 Devices):
RK3188 Devices suffer the same fate as RK3066 currently as they do not support hardware acceleration under the LINUX OS and therefore cannon playback video well with XBMC.
Quad Core Android XBMC boxes are going to be able to stream XBMC video with Android running in the background well. There is still quite a bit of development going on with these though.
If anyone is reading this and contemplating buying one, be very sure you trust the buyer. Many boxes may look the same but have very different chipsets. Unless these chipsets are configured with the correct kernel and drivers, you will notice all videos either running poorly or dropping frames every few seconds. With a perfect configuration, 1080p videos should run flawlessly with the Android system.
There are only a couple devices out that have the correct chipsets regardless of what they look like. For XBMC, these devices will still need that to have the correct software, drivers and kernel to make everything work properly.