Since 80% of a window is glass, it is always wise to pick the right glass for the direction the window faces in your home.  LowE 366 Argon filled glass for the south and west exposures may be the best fit.  LowE 272 Argon filled glass for the north and east exposures might save you money and be just fine in those locations.  Visable light will be something you want to check if you have plants that require lots of light to grow.  The higher the number the more light for the plants.  Any window that has a U value of .27 or lower will be very energy efficient in both the summer and the winter.  Double pane vs. triple pane may make a difference, so be sure you review the manufacturers specifications before you make that decision.

The other decision you have to make is what kind of frame you want the window made of.  Vinyl, wood, wood clad and fiberglass are the main options you will have to consider.  Each has their plus and minus points.  However, as I stated above, 80% of a window is glass and most quality windows have the same glass options from the same manufacturers, so be sure you pick the right glass with a low U value and you should be happy.

Also, keep in mind that installation of the window is just as important as the quality of the window and the glass that is in it.  I advise you to get two or three bids and check each contractor out to make sure they have happy window clients.