The XP rewards vary by level (and each mission can be attempted on a variety of difficulty levels). As for the XP rewards themselves, they can only be awarded once per mission, so repeating them on the same difficulty will not yield an XP reward (you can note this by hovering the mission, the rewards section will not include the XP bonus).
The bonus is somewhere in the range of 1k xp for levels 1-5 (early earth), 2k xp for levels 5-10 (earth and moon), 3k xp for levels 10-15 (moon and venus), and 4k xp for levels 15-20 (venus and mars).
Basically your best bet is to complete bounties and the story line. There is also another reason to do the story line while leveling: when you reach 20 you will not automatically unlock any of the missions. This means if you wish to do any bounties in the future or farm from specific places you will not be able to until you complete the missions - so they might as well get done while leveling.
It is best to complete the story line in order to unlock all of the missions. If you start from level 1 this will leave you at roughly level 17 when you finish the last mission on Mars. The last 3 levels may be completed by revisiting strikes, missions at their hardest available setting, or in the Crucible - but will be done fastest by doing the daily bounties.