Question: Which one is appropiate?

> 
> I will submit my assignments by tonight
> 
> 
> I will be submitting my assignments by tonight.
> 
> 
> 

I want to express that before the end of the day, I will submit my assignments.

Answer: 
I will submit my assignment by tonight.

I will be submitting my assignments by tonight.

Both the sentences express the future and are interchangeable, with a slight difference in meaning.

The first sentence formed with will + bare ininitive has the overtures of certainty, and intention and emphasizes the action. On the other hand, the second sentence formed with will be + present participle is merely a statement, which means that the assignments will be submitted in the normal course of events or a routine. It's therefore considered to be usual or not so promising as the first sentence.
1 will submit

2 will be submitting

In my view , the formula 2 here is not used for stressing the progressive aspect but simply to give the idea "will submit" more weight, simply because the formula "will be submitting" is longer. Whether this emphasis of the submitting act is appropriate or not is another question. I have the feeling "will be submitting" sounds a bit like a cheap rhetoric effect.