Well, not new to computers & BU strategies, but just obtained a new rMBP - this is my personal computer (in retirement) - files are not critical, so doing Time Machine on an external HD, AND also double BU of my Docs/Music/Photos onto a La Cie external SSD.
NOW, about to obtain a new iMac (mainly for the wife) - will do Time Machine onto a Seagate I TB HD and also Cabonite online - BOTTOM LINE - a single BU option is not enough for important files, AND if these are crucial then offline storage is a definite consideration (who knows, your house may burn down?) - 🙂