While double glazing and secondary glazing are quite similar in what they try to achieve, there is still a significant difference in how they go about it and the results. Personally, I believe double glazing is much more advisable but they both have their pros and cons.
Both of them can save you a lot of money on energy bills as they tend to trap heat better than single pane windows, but the biggest difference lies in their costs and effectiveness.
For starters, double glazing tends to be more effective but also more expensive. A double glazed window is about two times more effective than secondary glazing when it comes to heat regulation.
Conversely, secondary glazing is usually less expensive but also less effective. In the end, it all comes down to how much you’re willing to spend and how good a result you seek to achieve.