Also, the decision which one to use may relate to where the the intention/plan came from.
At least if someone has recommended you should buy a kanji dictionary, and asks you afterwards if you bought it, 買うつもり would be more natural (e.g. in 買うつもりですがまだです。), as using と思います might sound like you are not giving the person the proper credit for having given you the recommendation.
On the other hand, if you e.g. tell your friend that you have an intention to buy one, and want to ask his/her opinion, 買おうと思ってます might be better (e.g. 漢字の辞書を買おうと思っていますが、アマゾンで売ってないかな。), although I guess here つもり would be quite OK, too.