We'd suggest the Mondeo out of those two but they are both so good, you won't be disappointed with either. You can certainly get a good example for your budget and there's no shortage of Mondeos or Mazda 6s on the secondhand market. Your best place to look is either Ford/Mazda garages or car supermarkets. Part-exchanging your Mondeo is less hassle than selling privately and you will get less, but if you're buying something relatively new it often makes sense as you don't need to worry about advertising it, doing all the paperwork and then possibly having a period with no car (or indeed two cars...). You can wait until September when prices will come down slightly but the Mondeo and Mazda 6 are such popular cars there's unlikely to be much difference in choice whether you buy one now or wait.