Needs, spec, budget, resale, NCC or IVA approval, etc, all make a big difference.
In my case I was pretty uneducated on the subject so stuck to builders with NCC and IVA approval, which was pricy and limited choice somewhat. Nevertheless, if you want genuine quality, aftersales support and the reassurance a reputation going back decades it was the only way to do it.
Someone with greater knowledge than me can take a punt at a wider range of vehicles for perhaps a smaller budget and come away similarly happy.