uPVC is the most popular choice in the UK - and for good reason. It's outstanding value, superbly insulating and available in a huge range of colours and finishes.
Aluminium wins on strength and slimness. Its structural rigidity supports much larger panes of glass - ideal for bi-fold doors, roof lanterns and contemporary picture windows.
The best answer is often both: uPVC on the main house, aluminium on the extension or garden doors.