The ability to discern between 720p and 1080p is a natter of viewing distance and screen size. The chart below should tell you at what viewing distance a 42-incher should be so you can see some difference
If I were to interpret this chart, on a 42" screen size, a 720p picture has maximum benefit at around 8ft viewing distance.
But shorter than 8ft, a 1080p picture on a 42" starts to benefit the viewer and will have the maximum benefit around 5ft viewing distance.
This means at 8ft and above, you will not see any difference. but below 8ft, you can start seeing the superiority of 1080p over 720p in a 42-incher.
The implication here is that 720p will be just fine on a 42" TV if your viewing distance is at least 8ft away. Having a 1080p content won't make a difference at that distance.
But if your viewing distance is only 7ft or 6ft or 5ft, having a 1080p resolution will now matter.
Another implication here is that at 8ft viewing distance, if you plan to upgrade to a 50" TV, it should now be 1080p. Because 8ft is within the range where 1080p matters for a 50-incher. And if you upgrade to 64" you get the maximum benefit for 1080p. Larger than 64" and you will start to see pixels on the screen unless it is 4K.