You do not want to skimp on the chocolate chips. Nestle works the best. Do not get great value brands or even Hershey's. And make sure that you get semi-sweet, because milk chocolate will make your fudge way too sweet.
A trick is to put the butter in the pan, and then use the wrappers that the butter came in to butter the 9x13 pan.
This takes some practice, but the best results come when you buy one candy thermometer and always use the same thermometer so that you know its quirks. Each candy thermometer is calibrated a little bit differently, so just get to know yours.
If you have two pots and two candy thermometers, you can make both batches at the same time, and then swirl them together before you pour into the pan. Don't swirl two much, though, or you will end up with just one combined flavor instead of two flavors swirled together.