I would start with Rogue. Just because it's older and may be harder to go back to.

@meatball I think they're both dramatically different AC games where the order might not matter as much. All the pirate, ship based stuff carried over from Black Flag into Rogue has been completely axed in Syndicate and similarly Syndicate introduces new fighting and systems that are not present in Rogue. The only question is do you feel like doing a whole lot of sailing or a whole lot of running around London?

well Rouge ends where Unity starts so play Rogue first.

Yeah, this is definitely one of the big things that is making my decision so hard. They're quite different Assassin's Creed games to each other, and both offer things that I'm interested in doing. :P

@meatball: As far as I know, Rogue is the shorter of the two and doesn't have any follow-up DLC. So you can knock that one out a lot faster than Syndicate. They're skipping a year with these and apparently reworking the formula from the "ground up" so technically nothing from Syndicate will really carry over which means both of the games you want to play are book-ended on both sides from a continuity perspective.

Rogue is better, so start with that one.

I'd be a strong advocate for Syndicate. It stands as a very enjoyable game in its own right (the best since Brotherhood for me) with some great history-fiction, a great looking city to explore, and likable protagonists.

I haven't gotten very far into Syndicate but I liked Rogue a lot. It connects III, Black Flag and Unity, featuring characters from all of them. It's effectively half an AC story as it has 6 sequences where all the others (to my memory) have had 12. It still has a lot of side stuff though, so you can extend the experience as much as you feel like. It is very familiar in terms of mechanics, the new gimmick is an air rifle which is very similar to the blowpipe from Black Flag, so it might be good to play it before you retrain your brain for Syndicate's new mechanics.