the 2 tv's i'm talking about is the philips 42pfl9664 and 47pfl9664 the question myt sound daft but are you losing any quality from going bigger as the spec is exactly the same ?
On SD you will lose a little quality as the picture has to be blown up even larger in low resolution, for HD material you will probably not notice much difference though the extra size of the larger screen could be beneficial depending on how far you sit. Small screens usually look more "detailed" though you can't beat a good large picture, size is important!