Your question is more complex than you know. The most common eyepieces (Plossls, for example) have very short eye relief at short focal lengths, and a 6mm Plossl can be a pain to use. When combined with a 2x Barlow, the 12mm gives the same magnification but better eye relief than the 6mm. However, don't try to save money on a Barlow, or you will hate it. Buy a good one or don't buy at all. Each time you add a new lens, you increase the number of surfaces that can scatter light or warp the image, so high quality is a must. The Celestron Ultima is the least expensive Barlow I know of that is worth owning. Any Barlow or Barlow-like lens that TeleVue makes is good. Their Powermates, though expensive, are probably the best available, according to various reviews I've read.
Some high-end eyepieces (such as the 6mm and shorter Televue Radians) actually have a Barlow built into them to improve eye relief. They are superb eyepieces but cost about $250 each.
As you increase the magnification you'll worry less about wide angle--i.e. you want to see Jupiter at high power, or a double star, neither of which takes a superwide field of view.
In the end, the image can be no better than the lowest-quality element in the optical chain. A great Barlow can enhance a great eyepiece, but if either of them is poor, the combination is even worse. A good Barlow is the cheapest way to get a wide variety of magnifications. You can start with low and medium power eyepieces, and the Barlow then gives you two high powers.