significantly lighter at 655g compared with 760g (The Olympus lens also needs the additional weight of the 1.4x TC to get to 66-210mm f/4)
significantly shorter at 132mm compared with 160mm for the Olympus (and a touch longer when you add the 1.4x TC)
has optical image stabiliser (and when used on the latest Panasonic cameras 6.5EV Dual IS)
compatible with Panasonic's DFD AF system whereas the Olympus is not - ie. if you have a Panasnoic camera - buy the Panasonic lens instead of the Olympus lens - for Olympus camera users either will be fine.
the additional native 200mm focal reach of the Panasonic lens avoids the need to use a TC and thus allows wider apertures than f/4 as you zoom out (although realistically this is only of significant benefit below 100mm and it is unlikely you will be shooting at this for most sports)
the Panasonic is probably a better choice for the kids soccer games
BUT the compromises with the Panasonic lens are:
lens gets longer physically on zooming - this could risk water and dust entering as you zoom out despite the weathersealing
no MF clutch
no Lens Fn button
the Olympus has a slightly closer focus at 0.7m compared to 0.75m for the Panasonic
the Olympus lens provides f/2.8 all the way to 150mm which gives shallower depth of field (DOF) at 150mm and allows 1 stop lower ISO for sports at 150mm
Specs
50-200mm (100-400mm lens in full frame FOV terms)
widest aperture quickly drops from f/2.8 at 50mm to f/3.6 at 100mm, f/3.9 at 150mm