While any type of garage will technically serve its purpose, which in theory is protecting your vehicle but is usually is a receptacle for junk.Unless you are using it just to house your vintage 1964 1/2 mustang or a workshop with extremely loud power tools that will irritate your wife, I think that an attached garage is the only way to go.
It serves many if not all of the purposes of the modern garage. Number one, it will keep the weather off of your all important vehicles. That is the theory anyway. For most of us if we are lucky enough to have lets say a two car garage, then we are lucky if we can actually fit one vehicle in it. Most of the space is taken up with a plethora of useless stuff. Whether it is the out of season lawn furniture, or the fleet of lawn care tools that most of us will never use. I mean lets face it. Our yards encompass less than 1/4 of an acre, yet we have more power tool than the maintenance crew for Wrigley Field. Seriously, when is the last time you actually used that gas powered edger? Do you even have a garden to use that $1500 Troy Built tiller? That is with out even including all the things that your wife told you to do something with, which in your mind translated to put it in the garage. Us being men (for those of us that are) we don't think ahead enough to realize eventually we will have to clean out all of that mess. It would have been easier to listen to your wife and throw it away (actually if we listened to our wives more often, life would be much easier).
Let's switch over to never never land. You know, that mythical place where the garage is used for what it is supposed to be used for. It will keep your car snow free in the winter, and the seats won't be hot enough to sear the skin off the back of your legs. Even if you are not lucky enough to have a remote start, you can run out to the garage and start your car (and not worry about it disappearing), turn on the heater or A/C which ever the case may be at the time. This way when you leave for work with your coffee, cellphone,briefcase, and whatever else you remembered on the way out. your car is warm. More importantly, since you are without a doubt late, the engine is already warmed up and the windows won't be fogged up.
Beyond the conveniences of it being attached, there is one other attribute that I feel is the most important. That is safety. Being a male that is 5'11" tall and weighing 185 pounds (not to mention heavily tattooed), I am not so much part of the target group for criminals. My wife, on the other hand is a 5'4" blond that might weigh 110, she does fall into the criminals target range. With the garage attached, she can start her car, load her things, and by the time she hits the button on the sun visor, she is ready to "rock and roll". Leaving the garage isn't the part that I find all that important though. It is just more of a comfort.
Coming home to a dark house is where the attached garage really proves its worth. You hit the button, and the light comes on while the door opens. Before you ever leave your car the door has shut behind you. It is now relatively safe for you to exit your vehicle. There is no rush or the paranoid looking around and fumbling for the keys. You can take your time bringing your stuff in. Plus, you have the added bonus of just having to run to the garage if you forgot something. Which is you are a human, you will forget something more often than not, somehow it is just human nature.
I'm not saying that it is fail safe. Criminals often pick the garage as their point of entry. It is kind of like a deadbolt. It is just one more obstacle that might just make them go to the next house. That is not so good for your neighbor, but let's be realistic, you like the Jones but if it is you or them, them is always better. Hopefully some day human nature will change, but I would not bet the safety of my wife or myself on it changing anytime soon.
Another consideration to take into account that can either help or hurt is what it does to the value of your home. I will be the first to admit that tax codes and real estate laws are like ancient Greek to me. In some states and communities the value and therefore the property tax for your home depends on square footage among other things. A colleague of mine had a beautiful Asian style deck built on his house. He found out that this raised his yearly property tax. When he looked into it, he found that since the deck was attached to the house, it was figured into the square footage measurement, thus his home had grown. The city tax official told him if it had been slightly detached then it was a separate structure and different rules would apply. This may not be the case everywhere but it is some food for thought as to which style you choose.