I like the Metal Gear games way more than my impressions of Assassins Creed. Admittedly I have only played AC 2 and Black Flag, but I wouldn't say they're really comparable.

Assassin's Creed is about killing people in plain sight, in very populated areas. Metal Gear Solis isn't really about killing... it's about staying out of sight, from handful of people/things(cameras and such). It's also pretty linear to a degree. You don't have an open world to run around in. Mostly it's getting from Point A to Point B in the most efficient manner possible(and by efficient I mean the least amount of alerts). Now it's not a straight corridor, and there's backtracking and changes and such that occur. However most of the choices come from how you choose to deal with the guards. For example, from Point A to Point B may actually provide you with two paths. Each path leads to the same place, but has different security and guards that are doing different patrols. Even while stealthing, you have choices in how to dispatch with threats. You can get behind them and choke them out, or you can break their necks. You can leave the bodies there or choose to move them. You can also hold guards up by sneaking up behind them and pointing a gun to their heads, this nets a dogtag or other items/drops. Or you can just sneak around them via exploiting their patrols. It's a linear game, with a variety of approaches.

The Metal Gear games aren't designed in a way that will let you fight/gun your way through it. You have guns, but they're typically a last stand/boss sort of thing. More importantly, it's about sneaking past guards, avoiding cameras, hiding in cardboard boxes and looking at sexy posters while hiding in a locker.

Also, the story in Metal Gear isn't like any other games... seriously. It's insanely elaborate, confusing, and lengthy. The final cutscene in Metal Gear Solid 4 is I think over an hour long or something like that. That's just ONE cutscene... there are a lot of cutscenes like that. It's hit or miss. TBH I wouldn't bother trying to keep track of the story yourself beyond what's actually important to completing the game. It'll make more sense jumping on a wiki afterwards.

As for length... I don't feel they're short. Maybe if you skip all the cutscenes, but typically you don't do that. Metal Gear Solid 4 may be short, but I never felt like 1-3 were short at all.

Finally, the game has LOTS of easter eggs and it's one of those games where the Devs Think of Everything. It's a bit of a crazy sort of franchise, but for some very good reasons. I suggest trying the first if you can. If you can't stand the PS1 graphics there was a remake released on Gamecube of various reception. The graphics of the remake were good, but they had a new guy do the cinematics and some things are downright silly(like Snake jumping onto a missile fired at him and backflipping off of it). If you can't play the first, try the second. It's my favorite, personally. MG3 comes in a close second. The second one really stuck to the roots of the original and didn't really experiment much with the gameplay, contrary to the ones released after it, so I really enjoyed the simplicity of the game.