Well personally I would choose the 450D as it can space up to 10 fans (with 3 included) but it's up to you. I will link both cases on Corsair's website so you can have a look. Don't forget to choose a best solution
Obsidian 450D:http://www.corsair.com/en-gb/obsidian-series-450d-mid-tower-pc-case
500R:http://www.corsair.com/en-gb/carbide-series-500r-mid-tower-case