I had 2 t4s on my 2x4 to deal with the t5s I was running. They had to both run on 9 to keep the tent in the low 80s. I got a t8 and it runs on 2 to keep the tent the same, and does turn off a bit. It is so much quieter and worry free.
I would always agree to go big. You will love the ac infinity. Just follow the cleaning instructions and it should have a long life.