To choose the right phone/tablet is similar to choosing a PC.
It should be based on what you need it for.
Basics. CPU - processor - This is the brain.
Dual core/quad core/octa core - just means how many processors one CPU has.
You can have a super fast dual core 2.0 GHZ which will outperform an 1.0 GHZ octacore.
Sometimes they will market the phone as having Octa or Quad core but that wont mean that it is the better performing phone.
When choosing a CPU, of course we will check what type of C PU they are using. Just like PCS have mostly INTEL and AMD chips,
Smartphones/Tablets use Mediatek/Snapdragon/INTEL chips.
Snapdragons are used by Windows and Android devices. Intel Chips are used by all devices too.
RAM: 256 MB for basic function
512 ram - for browsing and video playback
1GB ram - mid range and for general use.
2GB ram and higher - Good for games and video editing
Some phones have LTE/and 64 bit capability.- I prefer those phones and you can have those phones for under 5k budget.
Storage or memory space is important as well. 16GB/32 GB is good.
8GB is standard for budget phones under 4k and 4gb and less - is for starter low end phones.
1 factor most people overlook is the GPU. GPU is like a video card. It is the CPU for the graphics. This is essential for gaming and video editing.
I prefer Good GPUS like the ones in the nexus 7 2nd gen and the K1/X1 Nvidia GPU used by Xiaomi mi3 and Xiaomi mi pad and Nvidia Tablet.
One way to compare the performance of your device is to install and run benchmark software like Antutu.