Have you used our free trial first to get a feel for the software? If you've downloaded the free version and had a go with it, and you like it, then the Pro version of Traffic Travis will be a big step up for you. There are no limitations on the number of projects you can set up (which makes it ideal for using with paid clients) and you get more keyword research, competition, and site health results. At the end of the day it's your call - if you only have a couple of sites, then you might be fine with the free version of Traffic Travis. However, if you are planning on expanding big-time then the Pro version offers so much more.
Feel free to shoot a question to the Traffic Travis support team if you need any help or further information as well.