Biggest difference between these two is that Hornbeam is completely deciduous - the Beech retains its dead leaves all winter. There are many options for hedging of the size you want, some of which are evergreen, though.

Yes I agree with bamboo though on one of the gardening shopping channels the lad held up the bare horn beam and said it kept its leaves whilst the beech next to him was resplendent in its gold livery. :o)

I also think the hornbeam has a wider range of offerings for the wildlife.

We were recommended to plant a hornbeam hedge instead of the beech we'd originally wanted because we had a very heavy clay soil in a garden we had 25 years ago.

Soil type is ueful to know. Hornbeam is better than beech on damp sites or more acid soils. I prefer it for its vigour, too, as it tends to grow outwards more, and (I think), make a denser hedge. Much as I love beech (I grew up near beech woods), I would go for hornbeam for a hedge on anything other than chalk soils. Worthy

I have both hornbeam and beech. Hornbeam is less fussy than beech, like Wothy says hornbeam will tolerate heavier, less free draining conditions but having said that, lots of wet clay here and both are doing fine. Don't wish to contradicte but my hornbeam retain their leaves to a certain extent, a bit curled and brown but still on - not such nice leaves like the beech, not glossy.