You tend to see T-tails on aircraft with aft mounted engines - it simply makes it simpler to place the engines if you put the horizontal tail on-top. However T-tails are far more susceptible to the uncoverable 'deep stall' phenomena. That's were the separated flow from a stalled wing effectively blanks out the tail so you can't push the nose back down to recover from the stall. Low tails are not normally susceptible to that 'deep stall' since at high angles of attack the horizontal tail is below the separated flow from the wing.