There’s actually 4 different FIANDRE jackets/jerseys/tops
The Fiandre NoRain jacket is the heaviest of them – for brutal/cold/wet conditions – full Windstopper with NoRain treatment, should be able to fit a jersey underneath as well as a baselayer.
The Fiandre Light WS Jacket – is mostly Windstopper but much lighter, same level of water/wind protection but not as warm, fits really well, more like a jersey
The Fiandre Light Wind Jersey has Windstopper front and arms with thermal no rain back, this one is like a long sleeve jersey and is really really comfy, not quite as water resistant as the two above but will keep you well protected.
Fiandre Light NoRain Top – this is more of a windproof shell layer, super light, windproof and water resistant, no pockets so best used over the top of a jersey, close fit too so pretty aero, in spring a short sleeve version will be in the shops too.
if you want to see some reviews check out Road.cc, RCUK, Cycling Weekly etc. all rate them pretty highly!
My favourite is the Wind Jersey or if its really grim out then the WS Light Jacket with a thermal jersey underneath.
Sportful and Castelli are both part of Manifattura Valcsimon but the all the staff including design, manufacturing etc are separate so you get different products.