Porcelain is a tough tile which will not crack or break easily. they are very long lasting. vinyl is out dated and just doesn't look good. Your other option should be ceramic tiles. So it should be between ceramic and porcelain. I'd still recommend porcelain but ceramic is cheaper, that's the biggest advantage of ceramic. I bought these porcelain tiles two weeks ago, they look lovely. beautiful hexagon porcelain tiles

Is does seem that the lesser known brands of LVT are the same thickness and same warranty as the major brands- the difference seems to be the colour choice and size of marketing budget. So as long as you like the colour choice with another brand then I think the cost saving is worth having. Separately the issue with herringbone over an expansive area is making the pattern meet up- generally a competent fitter will have no problems- I wonder if the issue is that you aren’t actually looking at LVT but instead floating floors (click together floors as opposed to stick down to the screed). Floating floors need to expand and over a large sized area can need an expansion strip adding which looks fine if you can add it to a doorway but looks odd in the middle of a large room.

We are coming to the end of a house renovation and we had wet UFH installed in all our rooms. It is a bungalow however, it is very unusual property with rooms below the bungalow so it is like a house in some respects. We have a mixture of solid floors and suspended timber floors and couldnt have a screeded system because of the weight. We had an ESP overlay board system installed and have found it to be very effective. Ours is run off an ASHP so I cannot tell you how it would work off gas central heating. Our UFH is on 24/7 controlled by thermostats in each room/area. We did install internal wall insulation on all of the external walls and window reveals and voids beneath the floorboards where possible and my understanding is that this is key to efficient and cost effective UFH.

I'd also be inclined to swap them as Jonathan suggested. Check fire regulations, as perhaps you do need to rotate the stairs to allow an open kitchen. (I'm in the Netherlands so can't advise on these.)