I am trekking since 10 years..i always found full track pants and t shirts comfortable.Jeans are a total no-no for me.And action trekking shoes always.
It is a very important thing trekking dress up is very much depends on the season and weather as well, like now days if you start trekking in north India it will be monsoon in few days and then you gonna need Quick dry Clothing and Shoes as well, and if you hiking up in the mountains you gonna need Warm waterproof cloths,or some extra pairs of cloths if you don't wanna bye new clothing. happy Climbing