That depends. If you require multiple entries and/or are planning on staying in Vietnam for longer than 30 days, the VOA will be the best option (as long as you are entering via one of the 3 international airports that have visa on arrival issuing capabilities).

If you only require a single entry and your stay will be less than 30 days, than e-Visa will be the better option, as you won't have to go through the visa issuing process once you arrive in Vietnam.