Probably the most important thing to decide is how many people are going to be in your group. There are advantages and disadvantages to solo touring, just as there are for traveling in a group. The biggest advantage of solo travel is that you make all the decisions yourself. So, if there's any question as to whether or not you will travel alone, you need to decide that first and foremost, because once you add another person, you have to share in the decision-making processes.
Sharing the decision-making, however, is not necessarily a bad thing. It is very easy, on a solo tour, to want to "see everything." With infinite flexibility, you can try to see every village, every ruin, every marketplace, every museum; you will not only fail, you will be utterly miserable and bored by the time you quit. The more people you have in your group, the more likely it is that somebody (or almost everybody) will not be interested in someplace you want to go. This can be a good thing, believe it or not. I spent two months in Greece trying to see every piece of ancient rubble, and it was a waste of five weeks. Three weeks would have been plenty, and it would have been possible, had somebody been there to veto my decision(s) to travel a zillion miles out of my way to see unrecognizable piles of stones in the middle of nowhere. Keep in mind the "two heads are better than one" idea, too: your friends may know something you don't, and you may see something really special that you would have missed without their input. One of my professors at the U of Oregon told me before I left that he wished he had spent more time in Turkey, which I had never even heard of and to which I otherwise never would have even thought to go. Turkey ended up being one of my favorite countries.
Though reasonably safe in some parts of the world, a woman travelling alone is almost always at risk. I hate to say it, but that's just the way the world works (at this point in time, anyway). Women are much more likely to be the targets of harassment, at least, if not abuse or violence. Even if she could ensure her safety, a woman traveling alone in many countries is not likely to enjoy herself. I have met many women travellers who have ended journeys early because they got fed up with being harassed. Women traveling together will be safer, and it is safer yet to travel with a man. Of course, men are not immune to harm. Before leaving on my current trip, I had my will drawn up, just in case. Men may be able to go places and do things which for women would be unsafe, but that does not mean we can throw our brains out the window and ignore our surroundings.
One of the wonderful things about traveling with other people is the ability to share the experience. When you come home, you will try to explain your experiences to your friends and family, and in all likelihood, you will encounter mostly blank stares. People simply don't understand. Even with the simplest of things, there's no substitute for being there. When you see a gigantic temple, and you marvel at the people who built it, it's much more satisfying to say "Wow" to somebody else, rather than just to yourself. You will never be able to convey that sense of awe with a picture or even a description to somebody who has never been there.
Additionally, solo touring can be quite lonely at times. To some degree, it depends on how much you like your own company and how much you need to socialize. It also depends on how good you are at making friends. I made numerous friends in Europe, but even so, it gets old having to explain to every person you meet that no, Oregon is not on the east coast of the US (it's on the west). It can be quite refreshing to have a somewhat deeper conversation with someone who already knows all the "basics" of your life.
On the other hand, traveling solo can provide you with experiences which simply wouldn't happen if you were with a group (particularly a large one). You are much more likely to strike up conversations with locals if you are by yourself. As a Canadian cyclist told me once, "Nobody's afraid of one guy on a bicycle." (Substitute "backpack" if that's your mode of travel.) If you are having a conversation with your friends, nobody is likely to feel comfortable butting in on your conversation. If, on the other hand, you are sitting quietly by yourself (particularly in countries where the color of your skin makes you obviously a foreigner), people will feel comfortable talking with you. They may invite you to sit at their table. They may even invite you to stay in their homes. It is a unique opportunity to really get to know the people and the culture.
A word of caution for those traveling in groups: a flexible itinerary can be disaster for a group. The more decisions you have to make, the more likely it is that there will be conflict, and as a result, unhappiness. The more you can make your decisions in advance, the fewer arguments you will have on the road. At the very least, decide in advance on a means of resolving arguments about where to go, when to go, how long to stay, what to see, etc. Whether it's the flip of a coin or a "majority wins" (dependent on the size of your group), it will save you an immense amount of grief if it's agreed upon in advance.