Look, honestly for private tutoring, you get what you pay for in most subject areas. For english, I highly recommend private tutoring. I had the most amazing english tutor; not only was she the best teacher, but she was so so dedicated to helping me succeed - took the time to mark as many essays etc in a very short period of time. I really doubt you get that kind of dedication + quality with centre based tutoring because they have to cater for the whole group. My tutor helped me to improve exponentially. Because of her, I ended up coming 4th in the year (which I never thought I could do) and getting 95 in the HSC. In year 11, I think I came around 10th.
As for maths, I think centre based coaching would be best. I had a private tutor for maths as well and it wasn't worth the money because for maths there is theory involved, but the key to succeeding is practice, which needs to be done alone. So if I could do things again, I'd do centre based because it would be cheaper and you could still get the necessary help + resources.