You do not provide enough information about your build and preferred playstyle for a qualified answer, but generally speaking I would take both and scrap the stealth pool.
If your concept is all about "being a SG leader" leadership should be a given. On the other hand, depending on the kind of team you normally end up in, you might get more mileage from the medicine pool... as I said, it´s hard to decide with that lack of information.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
I've found so far that the Medicine pool ( Aid Other and Aid Self ) is good both for soloing and teams, even unslotted.
Leadership is good as you can get Maneuvers which can get your teams defence up to 45% rather than the 40% from the FF buffs. This effectively doubles the effect of you force fields, reducing the chance of enenies hitting by half.
@Unthing ... Mostly on Union.
Knightly Defender is heavily depends on stealth. Thanks to Miss and Mr. Marvellous task force he learned the power of ghosting. I am doing my old missions on my own nowadays. Running to the key spot ( for clue, object or person and his guards) than if i like i pull them one by one and arrest the rest of thugs.
As you well know FF don't provide a good defense to defender while you are fighting. In my case my most strongest defences prevents me from attacking (PFF, Invisibility). With medicine pool i can heal myself when in my defensive stance. On the other hand, With Assault and Tactics Leadership pool will give me a big bonus on both acc and damage, which acc is the weakest spot of the Psychic Blast. And as stated by you, Maneuvers will add a big plus to my defence.
I hope this explanation will light up the weak spots of my question. For now my latest missions are in Croatoa. And here's a little detail of my powers;
Force Field: PFF, Deflection Shield, Force Bolt, Insulation Shield, Dispersion Bubble... Force Bubble will be added later.
Psychic Blast: Mental Blast, Psionic Lance, Psychic Scream, Will Domination... Psychic Wail will be added
Stealth: Grant Invisibility, Invisibility
Teleportation: Recall Friend, Teleport
Fitness: Swift, Health and Stamina
I also thinking about recpec this build just to swicth scream with TK blast. But not sure about that either. KB can be annoying for tankers or scrappers but it is good to save my own or other squishy team mates skins. Maybe i am wrong but i think also Psychic Scream is a great aggroing tool. As far as i noticed only a tanker can prevent mobs from running at me when i use that power. So it is also a life saver for others in many situations.
*sigh* Being a good hero is sooo hard.
My FF defender has both Leadership and Medicine. Maneuvers does make a difference to a team's survivability but there are still situations where someone gets into trouble and a little healing can make a huge difference in keeping them alive. My enjoyment of my character increased a lot when I added Medicine to my build - I felt more able to respond to situations as they arose. I'd probably have picked up Medicine anyway for Aid Self even if Maneuvers was enough mitigation by itself for the team - spamming Tenebrous Tentacles and Dark Pit does tend to draw a good share of the aggro my way 
Defiant 50�s: Generalissimo, Righteous Bob, Splortch, Brutus Cayuga
Union 50's: Chimera Obscura, Diet Anthracite, Grim Proctologist, Puny Little Minion, Raging Bitumen
In Soviet Russia, mission farm you!
Given what you say about build and playstyle, I'd say medicine would add more to your team.
Whilst Maneuvers will allow you to 'cap' your teammates defence, 3 slotted Grant Invisibilty will allow you to get really close ( 43% ) to the extent that anyone with Hover or combat jumping will get to 45%.
Just a warning, Medicine's interruptibility is irritating, not for Aid Self but for Aid Other. People tend to move about when they are low health, however you can't move when using Aid Other so it sometimes takes a while to land a heal.
@Unthing ... Mostly on Union.
Knightly Defender will hit 30 one of these days and i will pick my last pool power. He has Fitness, Stealth and Teleportation so far. For last pool i can't decide between Leadership and Medicine.
Leadership is good because he is a support character. Also it will give him many sweet bonuses to acc and damage which he is lacking.
As he's get hit many times Medicine will help him to keep on his feet. In PFF he can use Aid Self i guess.
I don't know how things will go after lvl 30 and which one will be more usefull to me. For concept, he is a leader of a SG. And as Medicine pool is using some kinda device both won't be a problem.
Thanks for your help.