The 690 II is good but you are restricted with RAM size if you want a RAD in the top as well as not being able to fit a 60m thick rad because of the heatsinks on the motherbaord. If you want a rad in the bottom you have to have a PSU that is no bigger than 160mm long because any longer and you can't get one in and even then it's still a tight fit and very cramped.
The Arc has much better air flow, fan options and room to fit any size RAM and there is clearance for the motherboard heatsink so you are able to fit a thick rad no probs. I know it's one rad vs two but the other options and the amount of space to play with are better than the ones in the 690 II. With a bit of modding there might be a way to get a second RAD in the Arc as well.
Other features the Arc has as well is USB 3, rubber grommets for cable management and better overall cable management and it looks better, IMO.
My $0.02