It is said that keywords selection is the core of SEO success. After recent algorithm changes, importance of keywords with tails has increased. So, what would you prefer to focus upon: short popular keywords or less competitive long phrases?   

short popular keywords or less competitive long phrases depend on the budget, if one has the money to run the necessary campaign short popular keyword will be away to go but is far easier to go for less competitive long phrases. Also is important for one to know what one is doing otherwise one would hurt the process.