does not matter technically, all kinds of dual class can give you the same passive skills for the main class. the question is, do you want to spend adena on additional set or weapon for a dual class? if you do, pick whatever you want, if you don't then pick a class that uses the same heavy armor (tank, tyrr) and weapon (tyrr likely).
you may want a dualclass that will allow you to have the mana consumption talisman. dunno if it is good for iss as it is for tanks/tyrrs.
iss already get that talisman (iss and wynn) as support, i think u would want to have dd as dual, so would recommend u a wynn. good to farm, for instances (ppl still like wynns in pt ), u can use your heavy with attack option and your iss weapon till u can buy blunt/bow. i think ppl play with main class more, so u wont want to spend a lot adena for dual sub to make it very good char if u can spend that adena for main. yes, tyrr would let u to use duals too, but i know ppl who changed tyrr from dual to wynn later, so make tyrr only if u sure that u like him, don't make for items only.