Need help in deciding which lens to buy?

I'm relatively new to photography, I know how to work my nikon d80 propelry, i know about iso and wb and what not, but i'm still learning the ropes when it comes to lenses. I take both landscapes and portraits, and want to try macro photography as well. My problem is this: for my birthday my husband bought me the sigma 30mm f1.4, which i tried this weekend and loved the way it shot in low light situations, but I already have a nikor 50 mm f 1.8. my question is this, will the sigma perform just as well as the nikkor? i'm not going to quarrel over a 1.4 and 1.8 bc its' not a huge difference at least to me. i guess what i'm asking is, is the sigma 30mm the same as the nikkor 50mm?