It depends on the size and lighting, as Tony said. Projectors are compared using four main factors: brightness, resolution, aspect ratio and weight.
Once these factors have been considered, you can narrow down choices having on account other factors such as contrast ratio, inputs and outputs, warranty, lamp life etc.
As you plan on using your projector on different places you might need to consider its weight. A lighter, more portable one -around 1 or 2 kg- will be better.
About aspect ratio most projectors, whether they are 4:3, 16:10 or 16:9, are compatible with other aspect ratios, but it´s not recommended to use a projector in a format other than its native aspect ratio as it will stretch or compress the image or it will miss parts of the picture. The best aspect ratio depends on the source. 4:3 is most of the times the best for using with computers. This depends on the aspect ratio of the computer you´re using, anyway.
Projectors work scaling different resolutions, converting different input resolutions to the native output resolution.
Scaling causes a loss of picture quality, tho. This happens not only when the projector´s resolution is lower than the source but when the projector´s resolution is higher too. Therefore you should match the projector resolution to the source's resolution.
Brightness is important. If you´re gonna use the projector on bright places you should choose one over 3000 lumens. Same for contrast ratio; the higher, the better for brighter places. Make sure the inputs and outputs are compatible with your computer too.