Okay...bear in mind that everything I say is entirely based on my own experience.
IMO pure PYP doesn't necessarily work that well, especially if your DC are ever likely to move into an NC system. It encourages a lot of independent thought and cross curricular links but can be too child-directed, at the expense of important skills without a good scheme of work.
It needs strong, rounded teachers, with an excellent communication framework, a commitment to long-term planning and very good evaluative skills. It relies very much on each individual teacher's skills set and experience and it's easy for teachers to duck out of teaching things they don't like/aren't confident in. I've seen it done terribly and I've seen it done well, but overall I would say the doing well is the exception than the norm. On balance I would prefer a NC framework at primary level.
Another problem/advantage with IB is the different expectations/training that staff have. You get input from a wide range of cultures and learning approaches, which is great, but there's very little consistent levelling/expectation of acheivement as each teacher will expect the level children in their country attain.
I like the units of enquiry and the holistic approach - focusing on the whole child rather than very specific skills - and it can work well. Which schools are you considering? How experienced is the PYP school at delivering the programme? Where do they draw their teachers from?
I've copied some stuff I've said on another thread:
IF WE ROLLED IN IB FROM PRIMARY UP
"I don't agree with PYP because of the lack of structure and content. I can see why you can't prescribe content throughout the world, and it may just have been the schools I've worked with and the way they delivered it, but the children didn't really seem to learn anything concrete. There was a lot of play and not much learning and I was looking after 2 children learning the same topic at the same time, with scant differentiation between them. They come out with interesting facts but not sufficient to build a foundation for future learning and the 9 year old who had been at the school from the very beginning struggled with basic maths concepts because they'd just missed out teaching them and the next year's teacher assumed they'd covered it. The whole class spend the second part of the first term learning stuff they should have covered lower down the school and that's a major flaw IMO. The teachers seemed to have too much control over what they wanted to teach and for it to work successfully you need a very international mix, not only of children as Bonsoir says, but of staff as well. The idea that the children lead isn't quite correct - the teacher still has control of the curriculum - but a good teacher has the freedom, under the PYP programme, to do what you said. Most don't.
Native language teachers isn't a fixture of the PYP, in fact plenty of schools do it, including one independent primary and the independent secondary I went to. It's more a benefit of being in an international environment and having access to them.
The primary NC is about to see major changes as a result of the Rose report and a move away from the overly strucutred and levelled system we have to something where teachers have more control over what is taught and how which is, I think, working towards the ideal. I do recommend you read the Rose report if you're interested in primary education because it's very accessible and recommends that we encourage more of the qualities you mentioned in your OP in the classroom. Cross-disciplinary learning is also a key feature of that and there is already a shift, in certain primaries, away from a subject centred approach (now we will do maths, now we will do history, now we will do science) to a topic centred approach and picking a topic such as the Egyptians and incorporating maths, science, history, art, language, geography etc which is one of the good things about a corrently implemented PYP.
MYP is good (but it doesn't give sufficient preparation for formal assessment IMO) and the IB diploma programme is excellent, although the levelling you refer to does still exist in that system (diploma). You have to tick all the boxes to move from 1 to 2 to 3 and so on up to 7.
I guess what I'm trying to say is the philosophy behind PYP is good, the intentions are there, the multi-disciplinary learning approach is beneficial but what it actually lacks is.....a curriculum. You can lay the principles onto a curriuclum and it will be fantastic but you couldn't abolish the NC and say 'this is how we will now learn' because the PYP is a 'how' and not a 'what' and when it falls down it really falls down on the 'what'. "