Kitaca (Sapporo city), nimoca (Hakodate city and north-eastern Kyushu region), Suica (Kanto and Tohoku regions, and Niigata prefecture), PASMO (Kanto region), manaka (Nagoya area), TOICA (Tokkai region), PiTaPa (Kinki and Hokuriku regions, and Shizuoka city (apparently?)), ICOCA (Kansai, Chugoku, Hokuriku, and maybe Shikoku regions), はやかけん（Hayakaken） (Fukuoka city), and SUGOCA (Kyushu region) are the cards that can be used nationwide.
However, compatibility issues might exist when using outside of the original region (I heard that charging is a big issue, but I had no problems charging my Suica card in Osaka and Sapporo at all).
The nice thing about the Sapica card is that it has a point system out of the box, and most people in Sapporo only use public transportation within Sapporo, so most people there use a Sapica card.
The point system is from what I know by myself, the rest is from what I heard from someone who has lived in many places throughout Japan before, please don't hit me!
Editing my answer to provide a more direct answer to your question, as I realised that while my answer does clarify your worries, it's still not a hard answer to your question.
When I was in Hokkaido I just used my own Suica card, and I was able to use it on both Sapica and Kitaca networks without any problems, same goes for vending machines, I only had 1 store where I couldn't pay with Suica (but only with Sapica), but elsewhere throughout the island was working.
Taking a look at the official websites of both cards, Sapica is strictly for use with the Sapica network only, while Kitaca can be used on both Sapica and Kitaca networks, as well as any other network nationwide.
However, stores in Sapporo might accept either both or only one of them, and if a store only supports Sapica, you can't use Kitaca, and vice versa.