IIRC the pioneer has a separate carrycot while the Fox and Peach use the same “frame”, but you use different fabrics for the carrycot and seat.
The only one I’ve actually had experience with off road is the pioneer and it was very smooth to steer on grass, had a good adjustable handle etc. It also felt robust which was good, and the car seat that comes with it (simplicity) is actually one of the longest lasting infant seats!
The older bugaboo Fox was apparently difficult to fold but they have resolved this with the Fox2. The seat is very high up which is good too. Basket space is good and it felt rather solid. I thought I would find it hard to push as only 5” but it was surprisingly easy!
They’re also bringing out a new peach, the peach 6. In the peach 5 I remember that there was an all terrain version too which had better wheels for off roading. I have seen the peach 5 for £700 which is a bargain!
The peach can convert to a double and I really like the double version of the peach, seats are the same size and it takes up the footprint of a single. The newer version actually has a built in buggy board too so you don’t have to faff around with them (although the bugaboo one is actually okay).
Another thing with the peach is they have elevators so you can have DC higher up which can be good if you’re tall, and DC can see things better on walks.
Another pushchair to consider would also be the Uppababy vista. Similar to the peach and can convert to a double (although has a larger footprint as a double). Large basket space (I’ve heard of people managing to fit a car seat in the basket. When I tried out in store managed to fit the pushchair seat in the basket!). Although it does feel rather tank like 
In terms of car seat a lot of them can fit on all the pushchairs if you buy the maxi Cosi/universal adaptors they can also take BeSafe, Cybex, Nuna and Joie car seat.
Just make sure to get the one that fits your car best 