How long is Assassin's Creed Brotherhood compared to 2?

So, every time I thought Assassin's Creed II was going to end, it just kept going to epic proportions.

I'm currently waiting for Steam to download Assassin's Creed: Brotherhood, and I'm wondering if I should expect a similarly-long main story line, or is it shorter/longer?

Let me say this, I'm in the middle of my re-playthrough all the games (doing 100% everything). First Brotherhood and now currently on AC2 (Revelations I probably won't play again for a while). I'd say I'm very good at the games and going through Brotherhood was a breeze. AC2 on the other hand is taking a lot longer.

Factors for the lengthiness:

1. Missions in AC2 are much more involved
   It's typically faster and easier to "speed run" through the main missions unlocking all that you can, then buying the strongest weapons and backtrack finishing all other missions. Brotherhood's missions were relatively linear. You head in one direction to take out a target and you move on to the next mission. AC2 had a lot of missions where you kill one target in one part of the city and you have to kill more at other parts of the city (all within a single mission).

2. You have limited "fast-travel" options
   In Brotherhood, there was just one large map with tunnels giving you quick access to most regions. You can also ride horses within the city. You can only fast-travel between cities, not within a city. And you are only taken to the outer walls of the city (or in front of Leonardo's). You have to walk/run to your destination.

3. Brotherhood only had one large city map
   If you're aiming for 100% synchronization, be prepared to travel between cities a lot in AC2 (and there's a lot of them). Coupled with the slower traveling options, you'll be doing a lot of running too.

4. It is incredibly easy to earn money and supplies in Brotherhood
   Just about anything you can do will get you money or supplies. Looting not only gives you money like in AC2, but you also get a higher chance at supplies (medicine, knives, smoke bombs, etc.). AC2 has a considerably lower chance if any. Chests give you items as well as the money in Brotherhood. And of course there's Assassin missions for you and your recruits. There's more to rebuild and earn money from (with multiple pickup points) in Brotherhood. You only have Monteriggioni to upgrade and only had one place to pickup.

5. Brotherhood has more options to kill/distract with
   The added items makes fighting so much easier. Particularly ranged weapons. You have more options to take out targets at a longer range. Poison darts, crossbows, multiple throwing knives, calling assassins. You don't even need to get your hands dirty. You can incapacitate more enemies with smoke bombs. You can only drop at your feet in AC2. You can throw your bombs at targets. After rebuilding enough of the city, there are more factions you can hire to help out just about everywhere in Brotherhood. They're hard to find in AC2. Combat in general is much more difficult in AC2. Counter's and disarms aren't very reliable once you reach higher level enemies. At least in Brotherhood, you could kick just about anyone and finish them off.

6. Brotherhood has kill streaks
   This is the big one here. You cannot kill large groups of enemies quickly in AC2 as you can in Brotherhood. You are forced to use other means to get kills quickly (grab then kill, knockdown then hidden blade, smoke then hidden blade, etc.).

There's a lot more that I'm missing I'm sure but these are pretty major.

The main storyline is to my experience, a bit longer, although the open world is smaller. And if you try to finish all the optional content and the secondary objectives (which are a new feature) on all the missions, then it's just going to take forever.

There are more sections featuring Desmond as well, as part of the storyline.