It really depends on how long you plan to keep the tv. Certain manufacturers, Sony for example, don't even give you the option anymore at these sizes. Panasonic are almost there too, all their 46" tv's are 1080p and they only have one 'budget' 50" in the TH50PX8 which lacks other important features such as the latest Vreal3 processor. It would seem that at these sizes, 1080p screens will soon be the only choice available.
However, if your question is more 'is 1080p necessary to get the best picture', the honest answer is 'not necessarily' as proven by the excellent non-Full HD plasma tv's from Pioneer.