Is it worth getting expensive windows fitted?

Whether you opt for PVC (entry level), aluminium or aluclad (top end) the u values (insulation measure) remain the same, depending on spec. In general if budget is a concern I advise clients to fit aluclad or aluminium in a sun lounge & surrounding area as the glazing bars are thinner (offering more light & views) and the doors are better quality. These are the windows and touch points most in use. Bedroom, bathroom widows and the like can be pvc from the same supplier (if they offer this) which saves money on low profile areas. Be prepared to invest in a good quality front door also for this reason.
I would certainly advise clients to move away form the grey windows that were very popular in the past. I think this has been overdone and the other issue is that a dark colour internally tends to frame your view where a lighter colour disappears and the view can speak for itself