Definitely Google Nexus 10 by MILES. It has the highest resolution display(2560x1500 pixels), a beast Mali-T604 GPU, a dual-core Exynos 5250 1.7GHz Cortex-A15 CPU and 2GB of RAM, 9000mAh battery, plus it has better 5MP camera, better performance, better gaming capability, and the latest Android software updates from Google.
Compared to Samsung GALAXY Tab 3 that only comes with a rather low-res 1024x600 pixel display, a weaker processor, weaker GPU, weaker 3.15MP camera, and smaller 4000mAh battery.
So it’s just no brainer that Google Nexus 10 is way better than Samsung GALAXY Tab 3.
In fact, just so you know, Marquees Brownlee, one of the most popular YOutubers(just did an interview with Motorola CEO), loves Google Nexus 10 a lot.
Hope I help you.