The purpose of an AI, aromatase inhibitor is to control the conversion of test to estrogen. On cycle is the ideal time to use an AI and your test levels are very high and conversion is therefore increased. On the other hand, a SERM, or selective estrogen receptor modulators does not stop estrogen from forming, but limits it's ability to attach to specific receptors. This means that while taking nolvadex, you may not get gyno, but your estrogen can still be very high, which can effect you in other ways. 

The key with using an AI on cycle is balance. Not too much but not too little. You don't want to lower estrogen to the point that low estrogen causes sides, but you don't want to let it go too high. The sure fire way to know what your levels are, is through blood work.