Nair and dair both deal 12% (7 frames of shield stun), but dair has 9 frames of L-cancelled landing lag while nair only has 7.
I use 3 general aerial heights when pressuring:
1. Aerial hits at about the peak of my SH, FF after hit. This isn't safe vs. Fox's shine OoS, Sheik's nair OoS, certain up-Bs OoS, and other quick options. Grabs might actually be possible some of the time, but generally I find people do not go for grabs either way because of the risk, and doing the aerial a little earlier than guaranteed or just within guaranteed safety discourages them from trying to move OoS or forces them to shield initially instead of dashing away.
2. Aerial hits late enough that the opponent is in shield stun the entire time between the aerial hit and shine. It can be done either with or without a FF. You must start the aerial before the peak of your SH if you plan on FFing, or else the hitbox won't come out in time. I generally use this without FFing when I know they are scared (high %s), and I milk the time for as much shield damage as possible before the super late aerial. With the damage from them holding shield while you're SHing + the aerial shield damage + shine shield damage + shield damage from an early aerial after the shine, you can wreck shields and often just get outright pokes. I do super late bair-shine-bairs and get pokes on people all the time at high %s. Also a great pressure option when you think they will hold shield and are fine with just shine grabbing.
3. The most "medium" pressure option (and imo the most difficult to grasp/execute) is done by starting the aerial before the peak of your jump, FFing before the aerial connects (keeping in mind you can't FF until you've reached the peak of your SH), and having the hitbox connect after you've started FFing. This beats grabs and even the faster OoS options, but leaves enough time for them to get caught by your shine if they attempt to do something (unlike #2 where they are locked in shield). FFing has a 3-frame buffer which can help, but you cannot FF during hitlag so getting the FF before the aerial connects is a great way to ensure consistency in your pressure. If you've ever gone from playing mediocre/bad players or good players whose chars don't have great OoS options to playing a good Fox, you may have experienced the "OMG, I'm getting shined OoS constantly" effect. It generally doesn't mean your pressure has gotten worse, but rather you were using #1 pressure vs. #3.
Kadano info on nair-shine pressure with amazing GIF that you should watch frame by frame until the image is burned into your mind.
http://gfycat.com/WarlikeWindingBichonfrise
Sources if you'd like to do calculations of your own:
http://www.mediafire.com/view/kwbkgsdok1dekd7/SSBM_Hitboxes_(NTSC_1.0)_&_Knockback_1.5k.xlsx
http://smashboards.com/threads/frame-advantage-on-block.309694/
http://smashboards.com/threads/falco-hitboxes-and-frame-data.300397/