heloc or home equity line for duple renovation?

Hey there guys, my fiance owns half a duplex, while her cousin owns the other half.

We want to move in to our side of the duplex, and since the house is paid off, we want to use a heloc or HE to renovate the duplex before we move in. 

I was reading that it is better to get a HE because of a fixed rate, but are there any other pros and cons? I also read that an HE will affect one's credit score tremendously.