Know any good trannies?

For crying out loud, I'm not taking about THAT kind of tranny.

I'm buying a 2011 tractor this weekend and I've got it narrowed down to either a D170 or an X300 series. The three tractors I'm looking at have three different trannies:

D170 (26 hp B&S, 54" cut, $2.8k) has a hydrostatic (they don't list the model)
X300 (18hp Kaw., 48" cut $3.4k) has a K46 automatic
X320 (22hp Kaw., 54" cut $4.4k) has a K58 automatic


What's the difference, and is one better than the other in terms of durability and pulling power?

I believe the D170 uses the K46.
The K46 is good for level ground and light towing.
The K58 is a step above the K46. It is good for mostly level ground and slight inclines. It can handle more towing over the K46.