Hi Lee,
First moved thread to this forum as it has lots of TV boxes in it
OK, being cheeky aside, from my experience of my wee crunching boxes
Don't use mine for TV so can't comment on that. Guess it depends on whether you wish to simply stream ( Kodi etc ) or whether you want a package ( Now TV etc. )
With a package you're stuck with the box they sell.
If you want your own, which to stream to is what you want, then the internet is your oyster and there are many to choose from. I have bought lots off this company
http://www.geekbuying.com/category/TV-Boxes-505/ never had an issue with their stock or service. Really recommend. Alternatively obviously Amazon.co.uk etc.
2 issues I have found with the boxes in general
There ability to pick up WIFI varies enormously. If you don't have a strong signal upstairs get one with an external antenna. If for watching TV , the streaming signal ( speed and stability ) is far more important than the crunching power. Doesn't matter how fast the box is, if the signal is crap then it will spend all it's time buffering.
If you are using to crunch then almost invariablly you will kill the psu. They are almost always 5v 2 amp. Replace them with a 2.5 or even better 3 amp psu and they will last forever. I've killed every original psu I've ever had on every single droid box. Also if it has a chinese made 2 pin to 3 pin adaptor, throw it straight in the bin and buy an eu marked shaver adaptor from Tesco's Asda etc. etc. They are generally poorly fitting and a complete fire hazard Certainly don't put them in the kids room.
Adding crunching to it is a simple stop at play store ( OTHER BETTER SERVICES ARE AVAILABLE like aptoid etc ) Install nativeBOINC ( android 4 or below ) or BOINC ( android 5 or above ) and off you go. Nothing to set up, mess with etc. Simples or simply give us a shout on here and one of us will walk you through the various options you can set. Surely that shield tablet of yours is crunching ?
For crunching 4 core RK3188 Cortex A9 based boxes are dirt cheap, bullet proof but not as fast and generally have android 4 or such on them.
8 core boxes you would think are faster generally but they also tend to have less memory per core so in the long term they are much the same. ( The difference between fast cortex chip and a slow one is nothing like as marked as between fast and slow cpu's ) An Cortex A53 is actually slower than a cortex A9, but it has 8 cores versus 4 so the amount of memory becomes critical.
Can't think of a single Boinc project that supports 64 bit yet.
Personally I think the RK3288 (Cortex A17) based boxes are the best compromise between power and price.
2 of my original gang of six cortex a9's are still crunching 24/7 , 365 days a year after 4 years racking up god only knows how many hours on WUProp for almost no lecky bill.
There are threads on here about the various chipsets. In particular read
this thread which compares the various chipsets and their relative power.
Remember what I said about memory. Most boxes have 1 or 2 GB and 1GB memory for 4 cores on a slower chipset will still outcrunch a faster chipset with 1 GB memory for 8 cores or you have to restrict the number of cores and hence you paid more to use 4 cores.