You question is too wide ranging for a simple answer. It would be nice to say that the more you pay the more secure the windows but this is not the case.
upvc windows are probably the least secure followed by wood and then aluminium. However any type of window not fitted correctly will be insecure, as will windows with locking stays which are not locked.
Bearing in mind that if the T man wants to break into your home he won't care about the amount of damage and will get in one way or another go for the windows which you think look good. Ask you neighbours for recommendations.
Get recommendations from your friends or neighbours. Big companies do tend to start thier prices high and will usually drop to at least half of what they first quote. My advice is to ask about and get a local well recommended company to do the work. You will find that they are less intense than the big companies and you would get a better standard of workmanship.
As for security, if someone is that determined to get in they will. But making it harder is what it's all about. Ask the salesman/owner of company for a few samples of the locking systems they use, and again have a good look at friends or neighbours.
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