My choice is BOTH. If budgets are a concern then lets look at the facts. It all depends on your windows and where they are.
If they are new and always stay locked you may do better with Glass Breaks. (go check if they are locked you might be surprised) If a burglar breaks your window to gain entry, the audible sensor in the glass break detects the intrusion and trips the alarm. If you have windows that stay locked or cant open you need Glass Breaks.
If your windows are occasionally unlocked you need window sensors. These detect the position of the window. Without this device you are vulnerable when the window is opened and there is no sensor to let you know. Yes, they can break out the glass and never move the window to gain entry. Usually this "brilliant" mistake nets you some DNA evidence.
How about the children that want to pass through the window to enter or exit the house. This is Why I said I prefer both.
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