I haven't heard of Sketchup before but I've been having a lot of fun using Adobe Illustrator lately. I was trying to find an alternative to my previous white-out + photocopy approach to sketching. Within a few days of installing the program I was able to put together a set of professional looking designs for a client that were easy to send out in pdf format. Being able to quickly draw variations made it especially handy for visual problem solving, determining scale, etc.