New glass or new double glazing unit?

[flummingbird]

My main front windows double glazing unit has blown and I need to replace it. One company says just replace the glass, the other says the units are approx 25 years old and new units will be much more energy efficient.

Does anyone know if the unit itself will make a lot of difference to the energy efficiency, or if just the glass makes the main difference?? Don't want to waste my money!!

Is it the whole window or just a small pane of it, if it's a lot of it I would replace the whole window but really it depends what you can afford, if it means going into debt, just replace the glass