That really depends on what you are talking about. If you are talking about the Radeon 57xx series vs. the 48xx series, the smaller memory bus of the 57xx's are mostly negated by the high clocks on the GDDR5, and they perform very very similarly to the older 48xx's. Its definitely better to go with the DX11 57xx's over the 48xx's if they are close in price. If the price is way different (say a 4850 for $90 vs. the 5750 for $130) then go for the cheaper:-)
As he said initially, it depends one the specific cards you're comparing, which is why he cited an example using the 57XX and 48XX cards. Those cards are among the very few models to which this question even applies at all.
In general, this is not a very good question to ask as memory bus speed and Direct X version support are completely independent of each other.
Quite right. Really, the 57xx vs 48xx series is the only one I can really think of right now that applies to this question, and really the bus width isn't what matters- its the performance. If you asked the question whether its better to get a higher performance card with dx10, or a bit less with dx11, then its a totally different question. Whatever the case, I would be going to DX11 now as it is starting to be more prevalent in the games being released, and most DX11 cards are similar in price (only slightly more) to the cards with similar performance with DX10 support.