Under 46" you'll never be able to tell the difference.
I have a Dell monitor which is 26 inches. Besides the computer, I thought I could add a tv to it but no luck. I was able to hook up a Sony Blu ray player and it is very clear. The monitor has 1680X1080 resolution which I assume will give all the quality the Blu ray can supply.
The little 19” TV I bought has only 780 resolution yet I am not sure I can tell a difference and they are literally set up side by side so I can watch TV and Computer at the same time. If forced to choose I would say the Dell is sharper but it is not an easy call.