Out of interest I looked at the LD site and the price does seem competitive against Brittany Ferries. However, the ship they use is smaller than those used by Brittany and even these have had problems with the rough weather recently.
I have used LD from Dieppe to Newhaven once. While there was nothing seriously wrong we have decided that - for us - the extra cost of Brittany is worth while.
The Bay of Biscay is notoriously rough when it is bad, and several otherwise 'good' sailors' on site here in Spain have reported feeling uncomfortable or worse on crossings in the last year. It depends on how good you think your 'sea=legs' are