Yes,for perfect circles,use a hole cutting bit for a drill. They are quick,simple,and cheap. For cutting simple windows i reccomend a jigsaw over a dremel anyday,but for the more intricate windows,a dremel is the only option.
Definitely dont go cordless though,its too much of a hassle,the battery will likely die within minutes grinding through hardened steel. Ive had my corded one for 5 years now and its still going strong.