I'll say stick with whatever OS your phone is running, unless it is a Windows phone.
Not a lot of choice if you have an iThing - a few different model iPads and thats it.
For Android though, the Galaxy tablets are nice, and I liked my LG G Pad 7.0 LTE (V410) after I put a pure Android release on it and got rid of the AT&T crapware.
Think the "killer device" for Android though are the sub-sub-notebooks on fleabay. If you like to tinker, get 2, they can often be made to run Debian or other "pure" Linux distributions.
Crap, just bought an AR, but this for $110 screams "buy me!"
Intel Netbook Smartab 10" HD IPS Touchscreen 64bit Quad-Core 32GB Webcams WiFi | eBay