What drives the relentless quest for new features?

My query, for anyone that has converted their library using one of those tools:

1. Which one is the best?
2. How much space can you save?
3. Can you notice any difference after converting?
4. How long does it take? (per TB)
5. Is it harsh on the processor?

Edit: Mainly for 1080p content and want to convert to h265 from a mix of formats

Edit: No GPU, i5 8th gen

For your self hosted server...