On a 15", below are the pros and cons of both the screens based on your usage:
720p screen
Pros:
- Decent Screen Experience. 720p is not Pixellated if you are going for a good brand like Sony or Dell.
- Texts are readable comfortably and without any eye strain in Office Documents
- Internet Surfing is a blessing in 720p as very few web pages are actually designed for 1080p
Cons:
- Contrast Ratios are pathetic
- Viewing Angles are terrible on some screens
1080p screen
Pros:
- Excellent Contrast Ratios. Blacks look really black in videos etc, hence the colors pop out very nice.
- Good Viewing Angles - Its not a IPS screen but 1080p screens from good brands are known to have very good viewing angles especially the Dell FHD
Cons:
- Text Size is so small, its difficult to read normal folders
- Internet Surfing is a pain as webpages are rendered in half of the space and you need to kepp doing pinch-zoom to get things right.
- Word Docs will have more work space but the text size makes it less efficient on a 15"
- Yeah, you can increase DPI, but i have read that increasing DPI makes the texts look pixellated, which defeats the purpose of getting a 1080p screen in the first place (Not Entirely Sure about this)
All Latest Laptops with Graphics Cores will be able to display Full HD on a Large Screen effortlessly (Does not matter if your laptop screen is 720p or 1080p). So, if you are going to connect the lappy to your TV for Movies and Videos and Use it mainly for surfing or office purposes, you should be well off with a 720p screen and give your eyes some life
Whenever you are in the party mood, you can set the TV and Home Theater On
The above is entirely based on all the things i have read when i was contemplating buying a Full HD screen for my laptop and ended up keeping my 720p and use my projector for movies.