It depends if you plan to go back and maintain the installation for the years to come. Get very few call back on Vaillant, and easy to access and work on these days. Worcester loads of call backs and more painful to service (If doing a proper full service, and more costly).
Prefer the Baxi (or should i say Potterton) Platinum range if looking for combis, have not had ANY issues at all and a proper warranty !! More expensive but get what you pay for. Veissman also do a wonderful guarantee not that they cover much), but have longer term reliability. Happy hunting.