best would be personal but either is ok. just make sure if you use LLC check your name address and social security number is on check. (ssn should be on personal check). did you know you can avoid sending a check and form, which can get lost in mail, by using IRS direct pay?
most states also have a means to pay and avoid writing a check and sending form.
in addition most sites allow you to specify the date the $ is to come out of your account. you can also print a receipt which will have a unique identifying number.