A feels more spacious. What is the square metre gain from A over B? B would give you a useful corner both inside and out. Entirely up to you. I am not fond of floorplans that are angled but that is a personal choice. Decide how you will use the space and which would be more useful to you. A corner is often great for a plant outside and a tv niche or a bookshelf. It depends on the size. A does look much more open though. Do some furniture mockups and see which one suits you better.

Agree with the previous comment regarding option 'A' giving you a more sense of space. A spacious kitchen enhances your home's functionality and comfort, providing ample room for cooking, socializing, and storage. First and foremost consider your style and what you're looking for with your project.

it all depends on internal configuration, and how the rest fo the house connects.

but with limited info I prefer A

Some things if consider:

How the sun will interact with the space - are these walls? Or glass? Will light enter and flood the rooms better with the corner space (B) than A? What orientation is this part of the house?

Will it bring more value to your time spent in the space e.g. it's a kitchen and our evenings or family dinner time (around sunset) are usually spent here.

Does the focus of the view change based on A or B? As in, are you looking at a brick wall vs a nice garden?

There are a lot more practical things to consider about the inside decor or its potential but ultimately, which one adds more value to your life (not value to the home).

Good luck!!

Turn the question around. Which piece of remaining garden do you prefer? The same comments apply. Leaving a more usable space may enhance the design.

You need to consider the proposed structural options.

What type of roof will you have? Some roofs will be much easier to adapt to each shape than others. There will be cost implications and also a greater tendency to future problems (leaks, guttering, gulleys, etc, etc) depending on your roofing decision.

What will your walls be made of? Straight lines are always significantly cheaper than corners, curves and diagonals.

If you have a large proportion of glass, modern frames can reduce those cost differences, but, if you are that close to your neighbours sightlines, I'm guessing that the side/diagonal walls will be looking out onto a very close fence/hedge/wall.

Have you asked your neighbour whether he/she fancies an extension as well? Sharing a wall would reduce costs significantly and the whole sightlines issue disappears completely.