Best answer by Mathieu
Never trade your phone in with Koodo, you will loose out of so much possible value. iPhones can easily be sold for a good amount on sites like Kijiji/eBay. You can probably easily get 400$ as long as it's in perfect condition (and if it's not, you really won't get much from Koodo). You could take the money you get from the sale of your 5S and use that towards the cost of the new phone.
Also, just so you understand that once you take a tab plus, you will be forced to take a tab plus plan, so take a look at those before you decide on the tab plus instead of the regular tab.