Tina - just to clarify, electrolyte drinks and sugary drinks are designed to provide two different things. The sugary ones, such as Lucozade, are designed to replace energy (although personally I find they give me awful stomach cramps). The zero calorie electrolyte drinks, either as liquids or tablets that you dissolve in water, are to replace salts (eg sodium and potassium). Electrolytes are useful on long or very hot runs, and they can help you to take on enough liquid as they are often easier to drink than plain water, and I find they have less of that 'sloshing' feeling in my stomach. A lot of them contain aspartame, which is worth avoiding, but if Zero is the High 5 one, which I think it is, it doesn't.
Sorry, bit of a ramble, but in summary, if you're happy with water, stick with it. If you're planning to run more than an hour and a half on a hot day, consider electrolytes to help with hydration. But I doubt if they'd have much effect on fat burning.