I have a two year old Trek 1.5 roadbike. Its not a superbike by any stretch of the imagination but it's good enough for me. I've been thinking about upgrading either the tyres or wheels to get a bit more speed. The wheels are some Bonty branded with Bonty tyres (700×25) and as they're both off a cheap bike I think they're going to be heavy.
What would make a bigger difference, new lighter wheels or new lighter narrower tyres?

If you called it by its proper name you might get an answer.
But anyway, just buy some decent quality tyres.

Well you may save 200g off the pair of tyres if you went for a decent race tyre.

Going for some light carbon tub wheels would save you nearer 1500g, so I'd say wheels, definitely.

You would look a bit of a cock going into a shop and asking if they have any 'Reynolds RZRs for darkside use' though.

I've been thinking about upgrading either the tyres or wheels to get a bit more speed

Pedalling harder usually works, other than that Michelin Pro Race 3's are your friend

Oh and i agree, drop the darkside crap, its lame as ****

Tough one.
You'll have to spend to get the weight down to notice it. I wouldn't bother unless the wheels are worn or you've got too good for them.
In any case if you got new wheels you'd have to justify them with decent tyres. So I'd try some good 23c's first. And BTW it's a myth that expensive tyres are fragile.

Depends on how much you want to spend. Tyres ovver more weight saving for £ and you may get better ride quality also.

Narrower tyres are lighter but less comfortable.

Conti GP4000, or any other good tyre will do. Just get whatever you can find with the most money off. You can usualy get a pair for £35 – £40 if you shop arround.

Get some lightweight tubes too.

Conti supersonic race 28 (or whatever theyre called) weigh about 50g each so thats another 100g off each end over cheep tubes. Yes they puncture 'more often' but IME I still oly had 2 flats in 3000miles of summer riding. Shop arround and you can get fhem for under a fiver each as well.

As for wheels, set your budget and take your choice, I'd look secondhand at something like the current or last model shimano ultegra (the ones before were a bit flexy apparently), or the perenial faviourite – Mavic kysriums in whatever flavour you can afford. You could go handbuilt, but IME factory ones are lighter for the money, look better and depite weighing 86kg I've never broken them even riding on some fairly shabby roads.

What sort of wheels are they? Could do worse than take them to a decent LBS and get them properly fettled. I use Conti GPs and like them, but any of the tyres mentioned above will be ok