Which 18-55 lens have you used? The newer 18-55 IS models are a *lot* better than the non-IS versions, although there's still a fair bit of barrel distortion and vignetting at the wider end, and CA gets a bit worse when you stop down.
But on the plus side, it's damn cheap, even cheaper than the 50/1.8ii, and overall IQ looks better than the sigma 18-50 f/3.5-5.6. (I can't find much on the 2.8-4.5)
Compare that to the 28/2.8, the prime is actually softer than the 18-55 IS at 28mm, has more CA, and barrels more than the zoom. But then the prime is a stop faster.
There's not much else around this extreme budget end of the market, unless you like Lensbabies.
So without scoring some extra money, i'd be getting the 18-55 (*with* IS, I or II) kit lens.
(The only other option is to save a bit more for the much-better efs 15-85, canon/tamron 17-55 f/2.8, ef 35/2, sigma 35/1.4 etc. Probably depends on how long your vouchers last).