The night flight (BA138) is currently a 773 with the new WTP cabin.
I find it relatively easy to get 6 hours of sleep in these seats and function effectively the next day.
The day flight (BA198) is currently a 3 class 772 with the old WTP cabin, so what you may miss is the better screen for the IFE (the WTP one is tiny), the food/drink is similar.
Ultimately it comes down to how much you value the £450.