A comparative essay should compare and contrast the subject. This means pointing out the differences and the similarities between the subjects. By definition it is okay to point out just the differences or just similarities in a comparative essay, but since you are going to be marked by a professor, it is best to play it safe and point out the similarities and the differences. If there is an absence of one, i.e. a lack find out similarities, then point that out. If there is a lack of differences then you are looking at the same object/thing.
Your word count defines how descriptive your language should be
When comparing two or more things there may be a need for a lot of descriptive wording. This may be necessary due to the nature of the things you are comparing. For example, comparing two flowers may need lots on descriptive language whereas comparing two ideas may not.
You should however, be very wary of the word count. A smaller word count may not leave much room for descriptive writing. If you have a very small and limited word count, then add in all your points and evidence first and work on the descriptive side of the process last. If your work still goes above the word count, then trim down the wordage over trimming down the points.
For example, you could turn this sentence,
“The iPad has 16GB of RAM, and a quad-core processor which is very fast,”
“The iPad has a very fast quad-core processor and 16GB of Ram.”
Over description is a grade killer
Too much descriptive text is going to lower your grade. This is not a creative piece of work, which means your use of descriptive language is only going to get you so far. Concentrate on making a few good points and backing them up with logic and evidence. The descriptive side of your work should be more of an afterthought that improves the quality of the reader’s experience.
Think of clarity and leave some of your complex phrases for creative writing
Again, this is not a creative piece of writing, and your thesis statement is not a place to get complex. It may need a bit of defining and may need them expanding and background, but that is a task for your introduction and your opening paragraphs, it is not the job of the thesis statement. The job of your thesis statement is to make a point and show the reader what the essay is all about.
Put your biggest issues first
If there is a stunningly large difference between the two (or more) items/things/subjects, then point them out first. Your professor will consider your writing to be weak if you leave the big issues until last.
Do I put in all the smaller details?
This is a tricky question because sometimes the smaller details and issues get you the points, but there are times when a massive difference or similarity is going to dominate your essay (maybe because it takes a lot of explaining or maybe because it is a vitally important factor). As a rule, you should keep your smaller details (similarities/differences) in your plan as notes and then add them in between the bigger issues where they fit comfortably.
Should you write a section where you list all the smaller differences and similarities? If you have the word count left over, and if you have made your biggest issues crystal clear and well defined, then it cannot hurt to write in a section where you list all the smaller differences and similarities, but do not go to overboard. When it comes to differences it is often easy to cast a broad net and start writing about things people do not care about, such as how a battery will last 0.0001 second longer on one device than another.
Does your comparative thesis have a point?
If it does, then can you fit that point into your thesis statement? Are you comparing two ideas for the hell of it, or are you comparing them to see which one you are going to use in future experiments? If there is a motivation beyond the fact that you have to write the essay (because your professor told you to), then feel free to add that reason/motivation into your thesis statement.