When should I take which Tychus build?

Hey guys, so in my group, I'm the guy who plays Tychus so when a tank needs to get busted, it's my job. I used to just use the Icy Veins build but I've started branching out recently. It seems like ol' Tychus basically has two builds - one oriented around his Overkill ability (the Icy Veins build) and one oriented around his minigun/basic attack and the grenade with percentage damage to finish kills. The Icy Veins build seems to be very reliable, but when should I switch to the basic attack build? When there's a thick frontline bruiser / tank and they didn't draft enough CC to stop my AAs? Tips appreciated. Thanks!