Emax @ Mirdif City Center is the biggest and best in Dubai and my go to place for fair prices on electronics. For computers, you can't beat the Computer Plaza @ Al Ain Center in Bur Dubai, but for everything else Emax is the best IMHO. Like someone mentioned above, DSF is always nice. Lots of freebees and package offers.
Gitex Shopper in the DWTC is way over hyped, having to deal with the frenzied crowds and odors for not much savings make for an unpleasant experience.
DSF is a good time to buy, as you'll find lots of promotions. However, don't get your hopes up for discounts. Many of the promotional deals involve throwing in free extras with your purchase. It's great if you don't already have 6 toasters :-D
There are a load of electronics stores in the Bur Dubai Area (near the HSBC Bank) and on the other side in Deira behind the Carlton Towers Hotel. I deal with a lot of them in my line of work and some are better than others. Make sure you go with someone, then ask the question where the item comes from and whether it is new? They get into a tonne of trouble if they are caught lying (which is reare). Ask to see the box and turn the item on and check it, 99% of the items are fine, perhaps 15-20% are grey imports but if you intend to take it home with you then it will be exactly the same!
It depends on the model, I bought an iPhone 6 and it was about 5% cheaper than the UK, I bought a Lumia as a gift and that was nearer 15% cheaper. iPhone 6 plus and the Galaxy note edge when I looked were marginally cheaper in the UK.