Ali Express is a leading marketplace for Chinese traders, and Worldwide Brands is a US based directory of certified drop shippers, wholesalers and manufacturers from around the world. Both appeal to the same customers – people looking to re-sell goods online, particularly on sites like Amazon and eBay – but they work quite differently.
Worldwide Brands is old, and established – it was set up in 1999 and is older than it's closest competitors like Salehoo and Doba. With Worldwide Brands you need to pay for membership, which is expensive, and you can then contact over 8,000 suppliers individually. You have to deal with them direct, not through the Worldwide Brands site.
Ali Express, much like it's competitor DHGate, allows you to search for products and make purchases on their site – without having to deal with each and every supplier individually. This makes it easier to buy from, but that also could mean that it's easier for your competitors as well and therefore many niches may be saturated.
Ali Express has the advantages that it's cheaper and easier, but Worldwide Brands is more reputable and much safer to buy from. It's also less competitive and there are many products that cannot be found elsewhere – I expect partly because Worldwide Brands is so expensive to join!