Its interesting about perception - chrism_scotland percieves them to be slightly better than Sigma, whereas I was the other way round - I'd usually consider a Sigma over a Tamron.
I think in this country its largely due to distribution. Tamron has a European office in Germany, but UK distribution, sales and advertising would by the UK distributor Intro2020 - and probably have less resources available to them to promote their products.
Sigma on the other hand has offices in the UK, Germany, France and Belgium, as well as distributors serving other countries, so I perceive it to be a much bigger company than Tamron - but then they also produce cameras and flashguns.