[updatedude]

SIC are super stylized versions of Kamen Rider stuff. Supposedly to be more in tune with Ishinomori's true macabre vision. So they're more monstrous and organic. Early SIC were essentially statues, and didn't get really articulated till around the Ryuki SICs. Until give or take the Den-O releases, SICs were notoriously fragile. As in, broken right out of the box. Unfortunately, I've had personal experience with that. But modern stuff are nicely durable. Also, they're significantly taller and bigger than Figuarts.

1) While SIC has it's long term fans, I'm going to say stuff like Souchaku Henshin and Figuarts are more preferred and mainstream. Not everyone can get behind the SIC redesigns (the earlier ones in particular were monstrous to the point of ugly). That said, I think the impression is that SIC are more high end. And since the modern stuff are actually durable, they're playable as well. SIC tend to come with a ton more accessories than Figuarts too.

2) Figuarts offers accuracy while SIC is stylized and more "mature". The scales are significantly different. Where Figuarts baaaarely count as 6" scale (if you pretend the Kamen Riders are short Japanese guys compared to your DC or Marvel guys), SIC are towering behemoths in comparison. I think they're 8 or 9 inches vs the Figuarts' barely 6 inches.

3) In terms of pricing, bear in mind that SIC tend to be sold in two packs. Even when they're not, they still come with components for a full second form. Take SIC Kabuto for instance, who comes with a giant shell for him to assume his beetle form.

SIC Skull's a regular 2 pack release along with FangJoker whilst Figuarts Skull's a web exclusive.

In terms of the figures seemingly differing only a little bit, it seems to me that's because modern SICs aren't as monstrous as the old stuff. Track down some pics of the older SIC, where they look almost unrecognizable to their original designs, then compare with the modern stuff, where they're simply more exaggerated.