The Seasonic Core series GC 650w is probably the better unit, but since there are no reviews it is hard to say because Seasonic has begun outsourcing their budget platforms to RSY, a lower quality chinese manufacturer. Without knowing whether these core series units are being built there as well, like the Seasonic S12III, which is terrible, much like the XFX XT unit that were built by Shenzhen Rui Sheng Yuan Technology and pretty much tanked the reputation of XFX who was known for using only Seasonic platforms.
Seasonic is a good PSU manufacturer, but not everything made by them or that has their name on it, is necessarily better or even as good as, many other products. As always, MODEL is the determining factor in quality.
In this case, I'd LIKE to say the Seasonic is the better choice, but without having any factual information based on a professional review, I simply can't. The CX650 I at least know is halfway decent, even though it's not a fantastic model.
If you want something undoubtedly good, and can afford it, then find a Corsair TX, HX, RMx, RMi or RM, or AX unit, or a Seasonic Focus, Focus plus or Prime unit, or even a Seasonic G or X series unit, or an Antec High current gamer, True power classic, Earthwatts Gold Pro, Edge or EVGA B2 (NOT B1 or B3), G2, G3, (NOT G5), P2 or T2 unit. FSP Hydro G and Hydro X are also good. Rosewill Capstone M series are good. Stick to one of those units and you'll have at least a very decent or in some cases exceptional power supply.
If those are not available or you cannot afford them, then the gray label Corsair CX and CXm units, generally specified as either 2015 or 2017 models, are better than just about anything else you will find at the same price point. In a pinch, the Seasonic S12II (NOT S12III) and M12II 620w models are acceptable even though they are an outdated design that lacks some modern protections and are group regulated which could be an issue on Haswell or newer Intel platforms. Easily solved by turning off the C6/C7 low power states in the BIOS but still old designs that are best avoided unless they are the best of what's available in a given region, and sometimes they are.
The Seasonic M12II 750 and 850w units, mostly not found anymore, are also good and are not the same platform as the 520w and 620w models.