The two sentences do have very similar meanings, and are largely interchangeable, but there is a slight difference in implication. Your comments indicate that you have not fully understood this first sentence. Let me restate: your two phrases convey the same fundamental information; they both indicate a possibility that the speaker will do some work. We can use either formulation and be understood, we can substitute one for the other and convey the same basic information. So in your comments you say
can't I say : "There is traffic on the highway so I might arrive late."
(instead of) "There's traffic on the highway so I might be arriving late"
and indeed you can, but you might chose one over the other for some reasons I will now explore. English (and imagine many other languages) has many ways of saying the same thing, it's good that you want to explore these possibilities, but there are not always fixed rules about when to use which possibility. Often we are dealing with matters of taste and personal habit. Consider just a simple active versus passive example:
John delivered the package.
versus
The package was delivered by John.
we could attempt to justify when to use one form rather than the other (in the active case John is the focus of attention, in the passive case it's the package), but there are also flavours of formal versus informal speech and stylistic preference. When I was working as an author of technical documents we were strictly instructed to use the active form, never the passive. So I do not believe that you will find a single, simple heuristic to determine which of your alternative forms to use. I am pretty confident that most native speakers and writers will on occasion use one when the other might be a better choice, and in doing so will not be considered ungrammatical.
Using your examples, I believe that the choice of one form over the other allows us to convey one additional piece of information: what controls the decision to work.
I might work more this week.
implies that it is my decision about how much time I spend working, I might work, I might not work; I will decide.
Whereas
I might be working more this week.
has an implication that some factor outside my control will determine that I may need to do additional work. To expand on factors: If you are an employee your employer might require you to extra work this week. If you are a farmer the weather might require you to do something extra. If you are a lawyer your trial might be rescheduled by a judge, you may need to do urgent preparation.
For example:
The hotel is fully booked so I expect to be working more this week.
Summary: The same basic information is communicated, but different formulations allow us to convey additional information. In this case, we can convey the difference between choosing to do more work and being compelled to do more work. In general the difference between formulations may simply be a matter of style. Sometimes, we may simply see one as more poetic than the other.
