Should upgrades in a clicker / incremental game have an additive or multiplicative effect?

I'm working on a clicker / incremental game and I'm wondering how to handle calculations for upgrades. I could add upgrades, where an upgrade to the production of resources providing a 30% bonus and a 40% upgrade combine for a total bonus of 70% (multiplier of 1.7). On the other hand, I could multiply the upgrade effects, so that the two upgrades combine for one 1.3 * 1.4 = 1.82 multiplier. This last technique seems more intuitive for players, but I'm just wondering if I'm missing some negative effects. Any thoughts / suggestions?