Hardboard or Plywood for under laminate flooring?

The Mrs wants some laminate flooring down in the newly finished kitchen / dining room. It is an extension and as such the floor isn't completely level (slopes slightly) and has a couple of different floor finishes at present. OSB boards and floor boards. Some of the OSB boards are slightly proud of its neighbour so rather than throwing down the underlay and then the laminate I thought I would board it first to try and help with the unevenness.
I'm looking for some guidance if anyone can help: I'm not sure of the difference between hardboard and plywood (apart from how it is manufactured thanks to Google). I reckon hardboard is more flexible than plywood. So should I be going with plywood? I don't want to raise the floor by too much so was thinking 4mm or there abouts. Is that enough?
Thanks.
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Steven Campbell wrote:

I used Vinyl tiles as packers doesn't rot and easy to cut with scissors
--
zaax

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The packing bit is probably the least of my worries, its the slightly uneven edges between the OSB boards and floor boards. Theses could be felt as ridges when I had the last lost of laminate down.
Thanks though as I hadn't really thought of vinyl tiles. I was just going to use old newspapers.
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Steven Campbell wrote:

Good tip, just what I need this afternoon to level off bits of wonky floor myself. Ta.
JGH
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Hardboard /is/ more flexible, but it's also cheaper.
How much do the proud boards stand out by? If it's only a mm or so, I think hardboard + laminate would be find.
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wrote:

Hardboard /is/ more flexible, but it's also cheaper.
How much do the proud boards stand out by? If it's only a mm or so, I think hardboard + laminate would be find.
--

Probably only proud by 2 - 3 mm at most.
I just thought hardboard would follow the contours too much and you would
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