If you're thinking about upgrading your laptop or Windows tablet PC, you may want to consider waiting for computers that will use the new Intel Haswell processor. We're finally about to see fast performance in mobile computers that have all-day battery endurance. These newer computers will get released this summer.
The Intel Haswell processor is a 4th-gen Core processor. It's much more power-efficient. Imagine the power of i5 but the battery life of Atom.
Right now, your options for all-day computing on a Windows platform are dual-core Atom-based mobile computers like Windows 8 tablets. While these devices perform very respectably for basic computing tasks, they don't compare to the processing speed that you'll find with ultrabooks computers equipped with Intel i3, i5, or i7 3rd-gen Core processors (Ivy Bridge). So, you have to make a compromise. Either get a small and light Atom-based PC that has really long battery life but slower performance, or grab a larger, heavier device that is fast, but doesn't last all day.
One of the first laptops to come with the new Intel Haswell processor could be a refreshed MacBook Air in the middle of June. I'm sure the Microsoft Surface Pro will also get a new processor this summer that will give it longer battery life.