Triple Glazing is currently more expensive to buy due to it being fairly new to the market. Double glazing is in basic terms two panes of glass, whereas triple glazing is three. The idea is that having the extra panel of glass it will cut down on noise pollution and reduce heat loss making them more efficient.
If you currently have double glazed windows that are working effectively and are in the style of frames you like , then upgrading them may not be cost effective , however if you currently have single glazing or old broken down double glazed windows, you may opt to upgrade to triple glazed windows, which will become more desirable in older properties over time. Having modern, well-fitted windows may add value to your current property.
If you decide to upgrade your double glazing for triple you can resell old double glazed panes on auction sites to gain some money back.