Plywood or OSB to replace old floorboards / insulation considerations

I have a couple of questions in relation to a plan to re-lay the floor of a first floor bedroom.

Firstly:

I'm planning to remove the existing floorboards and re-lay with bigger sheets, in the hope that this makes the floor feel a smoother/flatter, with less (hopefully no!) squeaking.

I've read around and can't decide between hardwood plywood sheets or OSB. Does anyone have any strong views on this? I'm happy for the extra expense of plywood, but if it doesn't make a difference and I can save a bit that would be good to know.

In either case I would be going with 18mm thickness. It's just a spare bedroom, so nothing incredibly heavy. That said, if people would recommend something thicker, do shout.

Secondly:

While I have the floorboards up, I was planning to lay some insulation down (probably fibreglass, possibly insulation board) just because the house is generally pretty cold, and I want to insulate as much as possible.

Below the joists is what I presume is the ceiling below. I presume it's not particularly strong, and it couldn't take a great deal of weight - but I wouldn't have thought that there should be any issues with laying down some insulation on top of that as the weight is presumably pretty negligible? If there are concerns I could staple a layer of chicken wire between the bottom of the joists, then put the insulation over that, thereby taking any weight off the ceiling - but obviously that would be a bit of a faff that it would be good to avoid if possible.

Does anyone have any experience of doing this?

Any insight greatly appreciated!

O

I’ve been using tongue and groove OSB3 for flooring and I’m happy with it.
Between floors in your own home, insulation doesn’t save you money unless one or other room is unheated (unoccupied). But you may want to have some acoustic insulation; if the floor is up anyway, go for it.
Beware that care is needed if electrical cables get buried under the insulation.

Noted, thanks for the advice endecotp.

18mm chipboard flooring is usual choice ,Screwed and glued, just drop 100mm Rockwool in before laying .