It's worth the extra £200 IMO.
The main thing you get is a 105 groupset instead of Tiagra, which means your shifters, gear changers, cranks etc are all better quality. Only the rear cogs are the same.
You also get proper 105 brakes instead of the unbranded bits of useless metal Trek supply with the lower end models.
You also get a carbon seatpost.
In short you get a package of upgrades which themselves would cost much more than £200 to upgrade later date. So get the 2.3.
But actually no . Get the 4.0 as you get a good quality carbon frame, which will be a marginally smoother ride but not make you go any fasterthan the aluminum counterpart.