I wouldn't worry too much about that - sometimes I can't tell which is better either, and I just tell the doctor that they are really close. If one has more haziness or blur around the letters, I say that one is slightly worse. Feel free to take your time on those and ask the doctor to repeat them if necessary. Once the doctor determines your prescription, he or she should test it by having you look at a chart through both lenses and making sure that you are able to read all numbers and letters clearly. If it is not as clear as you think it should be, let your doctor know and adjustments can be made until it is just right.
They will also run the same tests again just before your procedure to make sure there have been no changes - example, contact-wearers have to wear glasses for a number of days before the procedure since contact lenses change the shape of your eye, so their tests such as the ones that map out their eye, will likely have some changes from when they were initially tested to see if they were a good candidate.
Hope that helps

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