This year, the local magazine reaches all homes in the area. It would be a great opportunity to let people know the benefits of your business.
I want to ask, should the two sentences from above be connected into one with a comma, or is leaving as two separate sentences?
Which is grammatically correct?
It's much better to leave them as two separate sentences.
then, how about the use of "so" to connect it? Or is it still better to leave it as two sentences?
You could connect the sentences with "so." I personally prefer shorter sentences, but if you prefer a compound sentence -- then go for it.
The two sentences from above can be connected into one with a comma to make a compound sentence. This is grammatically correct and can help create a smoother flow of the information you are trying to convey. Here's the revised compound sentence:
"This year, the local magazine reaches all homes in the area, so it would be a great opportunity to let people know the benefits of your business."
By using a comma and the coordinating conjunction "so," you establish a cause-and-effect relationship between the two ideas. Keep in mind that this is just one possible way to revise the sentences, and other options might also work depending on the context and intention of your message.