The original Syndicate is largely open. Each nation/area on the world map has a unique mission (with goals like killing all enemy agents, assassinating someone or "persuading" people to join you with a special device), you conquer it by beating that one and then you can pick almost any adjacent area for the next mission. So while you always start in Europe and have to do the same missions at first you soon get to choose the order rather freely. But let's be honest the strategic portion is rather shallow, basically you can just choose the order of the missions and decide what to research, so all that really changes throughout playthroughs is what equipment you have available in which mission. If you want you can replay a mission you've beaten before by causing an uprising by raising the taxes and see what it plays like with more advanced equipment. But honestly, there's not that much replayability here.

Syndicate Wars provides a more linear and plot-driven campaign (two in fact as there's two factions to choose from). The order missions are activated in is largely pre-defined but often multiple missions will be unlocked at once so you can choose the order at times (and IIRC some of them are optional). Also the missions are more complex, more heavily scripted and generally more interesting. Also despite the linearity (or maybe because of it) there's more replayability in Syndicate Wars. In Syndicate 1 you can just sit back and watch the money raining down at you once you've conquered enough areas (time passes by on the strategic screen). In Syndicate Wars on the other hand you only get money by beating missions and in a few other ways (selling equipment you pick up from enemies or through a few other ways that you may want to discover yourself). Getting money requires more effort and the linearity means that it's more important what items you research at which point and due to the gameplay itself being a little bit more complex than in the original game having certain items at your disposal may largely change how you do missions. Additionally there's quite a few scripted events unrelated to the main mission goals which you may not notice at first (and may provide ways to make more money or acquire equipment you normally wouldn't have at that point yet). So Syndicate Wars does offer much more replayability, IMO.