In my experience of debating antis, I have come to find that there is no winning, even when you do win. If you make spelling or grammatical errors, they will jump all over them, calling you "illiterate."
If you write a well-reasoned piece that has no gramatical or spelling errors, they will accuse you of trying to appear superior. Never mind that it is a fact that your spelling and grammar may indeed be superior to theirs; they think it's a form of bigotry.
In short, most antis are not rational, reasonable people, and there is no sense in wasting time on them.
FWIW, first impressions are important, and so it's best to try and watch spelling and grammar as much as possible. However, the bottom line is still the logic, and I'd rather see a poorly executed logical piece than a perfectly written piece of illogical nonsense.
Uf korse, i cood speel lyke this hyar an thay"d prolly cawl me uh dum rednaik. Thu funni thang iz, hits reely hard two delibratlee missspeel and yewse orwfurl gramer whin yew has beed edumukated proply. Hits dun tuk me 5 minits tew write this hyar pagaraf!