If you can, make sure you have photos of what your bag will look like. Better yet, see if you can purchase it in person. Photos can give you a good idea of any extra features your choice may have and give you a good idea of what the color will look like.
Make sure to measure your laptop so you know the dimensions before you make your decision. You might also want to consider whether you want extra room for other electronics and papers or if you are just planning to carry your laptop.
Ask yourself how they will hold up after carrying heavy items. More padding can help avoid the soreness that comes with toting your laptop around, and if they are wider, they can help you avoid strain on your back.
Material and details
Finally, you’ll want to consider whether you need some extra pockets and what quality of material is best for you. Check to make sure you have enough space for other items you’ll take with you.