The non-modular PSU always performs better than a modular one (unfortunately) but fortunately, the 400r has plenty of room to fit big cables in the back so I would go with the Rosewill. However, Seasonic is a quality PSU brand and you can't go wrong with a semi-modular PSU either. XFX and Corsair are quality PSU brands, too (just putting it out there).
You NEED a quality PSU because they degrade the fastest and can blow up your system. You want a PSU that has headroom to breath but you don't want a overkill because if you only use 300w f.e. and get a 1000w PSU, it will degrade very fast and it will suck up more power than it needs to. So in short, get a PSU with 10 to 50w of headroom but use most of your PSUs capabilities to have the most efficient experience.
I'd have to take exception that a non modular is always better than a modular .... personally, I think 100% modular is silly. What's the sense of having a modular 24 pin cable or 8 pin EPS cable, not like ya ever not gonna plug those in, so why introduce another failure point and increased cost..... hybrid modular gives ya the best of both worlds.
Yep, not sure that was exactly what he meant, he may have been referring to the Capstone being the better PSU overall, which is true. But I jumped on to make the same correction that Jack did... modularity does not affect a PSU negatively in any way. That was just a myth perpetrated by PC Power and Cooling back in the day to sell PSUs.