Personally I would start with a diffy, simply because they are significantly cheeper. Buy a diffy and then save up for a sweet ashcatcher down the road imo.
After the Glass on Glass factor, a diffuser is the second most important aspect of a quality bong. Especially a RooR diffy. It will go a long way in making your hits smoother. Also as he said, a diffy is way cheaper. I would get they diffy first, than the ashcatcher. But hey if moneys no issue buy em both.
Thanks doodz. I went with the diffuser. I ordered a 3.2 RooR Little Sista w/ Rasta Logo and a diffuser off EDIT. Came out to $258.78 w/ shipping.