Barnes & Noble generally revs their eink line in the spring/early summer (let's say by May/June) and their tablet line in the fall. It's a pretty safe bet that there'll be an updated Glow in the spring/early summer time frame this year, with the new higher resolution eink screen and better lighting. Whether or not it will keep the physical buttons is unknown.
Unless you have to have it now, I'd wait. Or see if you can find a good deal on a refurbished or used Glow.