Souter never had the slightest history of being a conservative and was only chosen because he was a long time friend of he Bush family and "acceptable" to democrats and RINO Republicans. He was picked as a consensus candidate because GHW Bush didn't want another contentious Bork nomination like Reagan had. Bush never for a minute thought Souter was a Scalia conservative, he kind of hoped he Souter would be a O'Connor/Kennedy moderate, but even that turned out to be optimistic.
Gorsuch, on the other hand, is considered by every legal scholar as a model of conservative legal thought. Yes, you can cherry pick a couple rulings (out of hundreds) where he might appear more moderate. But even Scalia himself thought highly of Gorsuch. At the absolute worst Gorsuch might end up more like a Roberts, but not Souter.
Robert's background was significantly more conservative than Gorsuch. Roberts was a devout traditional Catholic who clerked for Rehnquist and whose wife was a member of Feminists for Life. Gorsuch attends the most liberal Episcopalian church in Boulder (with a rabid left-wing activist female "priest" as the pastor), clerked for Kennedy, and made it trendy to use female pronounces like "she" and "her" to refer to men who "feel" female.