What do you think will give better results: in-house marketing or hiring an advertising agency?

There is no better answer. However, I will personally for in-house because i will have the opportunity to train them further to do the things of great importance to the company but you cannot train staff from an external agency. Also having in-house reduces costs and the money you would have used to bring in an external agency could be spent other important things for your company even if it means giving out bonuses to motivate your staff

In house by all means. Digital marketing is taking over, the opportunity to engage customers and prospects is about conversations. Agency relationships are too structured for the digital age and too focused on briefs and creative turnaround. the new age of digital marketing relies on a company's narrative on subjects close to a customer's heart. This requires a profound understanding of a business' value proposition that can come from its most focused marketers (or at least should come). So many ideas and tactics play out in marketing that on a day to day basis, marketers need to pivot and use engaging relationships with customers to put their points across. All this points to having the right digital tools that help marketers reach out to customers directly, often and with as little process as possible. Agency relationships to me are moving into niche areas like strategy and creative support and even over there agility, testing and analytics plays a key role in being able to guide marketers achieve better results. For best results, move marketing in house slowly, build customer engagement capability and get your agencies focused on analysing results and coming up with presentment ideas....

The short answer to this is "it depends". If you have the man power, time, and capabilities - then an in-house marketing team would make more sense. Nevertheless, outsourcing your advertising needs can come with great benefit... For example, you can hire an ad agency to run your campaigns for 6 months - 1 year and then apply what you learned in-house and use it to empower your marketing department internally so you won't need to continue outsourcing. 

It depends both options have their benefits and disadvantages.

If you hire an in house team, you can train them further and get them know business closely. While agency do their stuff by their own.

But if you have no time, can give your work to agency so you can focus on your business and let the agency do the work for you.

It depends on the size of business and the industry itself. 

In-House marketing is best but now a days , its the era of Electronics Media, So making Web Site and advertize all the information to the global world as well hire an advertizing agency with nominal cost if possible well be best for increasing the sales. 

That totally depends! On the market, product, company organization, budget, company philosophy, size, many many factors. Either can work best in certain circumstances.