With out trying to explain the benefits over the HT18. its probably best to refer you to another forum where its members have done all the homework for you with pros and cons, sizes, how to with the why's and results etc. 3208 post of it so be prepared to have a long read.. but worth the effort if a cheap new proven upgrade over a tied HT18 is what you are intending.
http://www.patrol4x4.com/forum/engin...ay-td42-80484/