Since they are both modular units and XFX PSU's are made by Seasonic, really the only differences are the efficiency rating, the slight cable advantage, and extra 2 years of warranty (7 yrs vs 5 yrs for the XFX) of the Seasonic (branded) unit. Both have outstanding warranties. So if saving a little cash on the electric bill or the few extra connectors matter to you, get the Seasonic. If not, get the XFX...you really can't go wrong with either one.
I don't know your full system specs, but if you use pcpartpicker.com , it will calculate your power usage for you in the upper right corner of the screen as you add your parts. Once you have that total, i would add maybe 20-25%, as most power supplies rated efficiency is at roughly an 80% load. XFX and Seasonic both have 750 watt models available too (and higher, of course). Hope that helps!
I would say that the 650 watt models would be fine unless you're planning multiple video cards or a serious overclock. You have roughly 200-250 watts of headroom without OC'ing. If you're worried about it, bump up to the 750 watt models. I always go with more than i need (within reason), with future upgrades in mind. These are quality PSU's that should be good for many years..better to go with too much capacity than not enough.
Thing is, I won't upgrade in another 4-5 years... so the question is, will this PSU do it for my current system? (no overclock, crossfire...)
It will be fine, I would advise going with an 80 PLUS Gold with modular installation if you won't be upgrading for a few years. The PSU will generally perform better and save power vs the price difference of a cheaper one in electric bills.