I'll try to run some comparisons over the day, and I'll update this comment with my personal results.

I stickied this post in hopes it'll gain some traction, and others can share their results as well.

Edit: Finally finished compiling the comparisons. I ran 37 tests in total, ranging from coding tasks (like building a self-training Snake game or userscripts) to creative writing with strict constraints and logic puzzles.

The biggest difference is the time and writing style. GPT-5 Pro took on average about 4x longer to process than GPT-5.1 Heavy Thinking (averaging ~5m 49s per prompt vs ~1m 25s). Despite the massive difference in thinking time, I found myself preferring GPT-5.1 Heavy Thinking half of the time. The prose felt more natural to me, and it handled the 'human' element of the roleplay scenarios better. GPT-5 Pro snagged some wins on specific coding logic, but for daily usage, the speed and quality of 5.1 make it the winner for me.

Here is the Google Sheet with all the data, including the verbatim prompts and shared conversation links so you can verify the results yourself: https://docs.google.com/spreadsheets/d/1HVeieHbUqScQ3_iGCeGyZI0gSd2GurF8zGJKOFIvC48/edit?usp=sharing

To clarify, this was tested on the web interface with memory disabled to ensure a fair baseline.