It really depends on your voice force. Shure SM58 is a great mic, by having very small feedback, but it having problems with fidelity. Sennheiser has more fidelity, but it has a greater feedback on the high range. I like better Sennheiser how it sounds, it has power, it has fidelity, but on live performance, Shure SM58, has a better place on feedback. Even in front of speakers (and we have very powerfull speakers) it does not make any feedback. Also, you may consider Shure Beta58, which is similar with Sennheiser sound... I saw a lot of great artists using Sennheiser on live performance/concerts. So, you may try one of them on a music store, and make some comparison with other brands in front of a speaker.
Regards,
Vali