And remember that wood is an insulator, not a conductor. I'm not convinced that underfloor heating will ever be the most efficient solution for you - it might feel nice underfoot (slightly warm), but is unlikely to be the best way to heat the rooms - which may not necessarily be a big deal if all the rooms are bedrooms but think about it anyway.
I thought about this for my renovation recently but ended up using fancoil radiators upstairs, because there would have been too much work involved in reinforcing the floors. I ruled out deflector trays after careful consideration. The alternative was to fill in between the joists with timber trays, put in insulation, lay the pipes and lay screed on top of that, which would have placed a lot of stress on the joists (old 7" ones which I had decided not to replace).
It can be a tough call. How much heat do you want upstairs?