Is it better to quit a job before filing for divorce in terms of alimony?

Don't quit your job, at least not before talking to a lawyer. It might make more sense for you to wait for the company to fire you, for example for refusing to take an assignment that comes on short notice and which interferes with your ability to care for your baby.

Either way, the court is going to consider your ability to earn a living, as one of quite a few factors that go into alimony awards.  Quitting, I think, would probably not be a good idea here unless you have another job lined up, since your arguments about how little you will be able to earn as a working mother with an infant will be better received if you show a good faith effort to earn what you can.

On a practical level, a contested divorce can easily take a year or more before the final judgment, and even temporary support awards other than child support can take months.  Then, you would have to worry about whether your husband obeyed the order.  Quitting could leave you without any income at all, for several months.

Alimony is not solely based on your current income, but a variety of other factors, such as the length of the marriage, age,  and most importantly, health, future earning capacities, income and gender. If you are quitting a job becasue you now need to be at home and take care of your son this is understandable.  The fact that your husband was cheating is also a factor the court will look at.  I do not think that it is a bad idea to quit as taking care of your son is important.  I suggest however, that you do in fact hire a lawyer to help you maximize the amount of alimony that you are entitled to under the law in NY.

In taking a look at a soon-to-be ex-spouse's ability to support herself, court's do not look wholly or even mostly at what she's earning at the moment; after all, anyone can have a downturn in life. Instead, the court will look to your overall earning capacity, based on your education, field, age, health, past experience, positions, and salary. Thus, quiting before the divorce will do little, if anything, to improve your alimony. It could even hurt you, since courts do weight general equity or fairness. Right now, the balance is your favor based on what your write (he cheated while you were pregnant); why through away that good will with what could be considered a cynical and manipulative attempt to game the system?