As far as I'm aware, the Vaillant you mention has built-in hot water storage, whereas the Baxi does not - this probably accounts for the larger size of the Vaillant.
I've had a poor Baxi combi in the past and would never buy one again. As you say - everybody will have different experiences and therefore different preferences. If my only choice was the two you mention, then I'd go for the Vaillant.
For me, it was a toss-up between Vaillant and Worcester-Bosch - I went for the Worcester in the end, but I'm sure I would have been equally happy with the Vaillant.
Incidently - my WB 37CDi combi also has built-in hot water storage, but I never use it. I have some lengthy pipe-runs in my house and it makes no real difference for the time it takes for the hot water to get to the taps, so I'm happy to use less gas and live with the few extra seconds it takes.But what do i know