It would help to know more about how you're planning to play this character? Do you have a regular team? Will you be soloing a great deal? Things like that.
Storm/Psy is probably going to solo with considerably greater ease, while Kin may be more in demand for teams. Both can solo, of course, and both do provide great team support when played well, it's just a matter of what's the easiest fit for each activity. Also, if you're a newer player, you may have some trouble using Storm to its full potential in teams as poorly controlled KB often irritates melee artists.
Iscariot's Guide to the Tri-Form Warshade, version 2.1
I'm sorry that math > your paranoid delusions, but them's the breaks -- Nethergoat
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Well, I would say I solo and team about equal. I have a close friend that I Duo with a lot, and less occasionaly with SG mates. As for storm, i'm well aware of the knockbacks that can bother meleers. I've got several 50's and have been playing for almost 2 years. Usually my friend and I will try to compliment each others builds.
Okay, well, the "several 50s and two years" thing still doesn't tell me whether you feel comfortable using KB or not, but given the tone of your remarks I'll assume that you do or think you do. Do you have any experience playing Force Fields, Storm Summoning or Energy Blast? How comfortable do you feel with your ability to more or less precisely control your KB?
To be clear, I wasn't trying to insult you, but you have a low post count and I didn't want to assume experience you may or may not have had and your OP didn't provide that info. Sorry if you took offense.
It's probably more important to be aware of the potential problems than to actually have the skill anyway, so you're ahead of the eight ball there - you may or may not be able to use it to the fullest, but at least you know enough not to make people angry. If it were me, I'd probably go Storm/Psy given this choice, but I'm very comfortable with Storm.
If the aim is to complement your friend's build, then what is he/she playing?
That may be the more important question here. Or is he/she going to make something to complement you?
Iscariot's Guide to the Tri-Form Warshade, version 2.1
I'm sorry that math > your paranoid delusions, but them's the breaks -- Nethergoat
P.E.R.C. Rep for Liberty server
No offense taken 
I just post very rarely. I'm more of a forum lurker than a poster. I didnt see any guides pertaining to the info I was looking for, so I decided to post.
As for force field, no experience there, but one of my 50's is an earth/storm controller.
Granted as an earth/storm its very easy to avoid knockback issues with an aoe immobolize, which I would not have as a defender. So I imagine i'd need to work a different strategy. But am I comfortable and familiar with storm powers? Yeah, i'd say so.
Of course, I am open to suggestions on managing knockback without AoE immobolize. Also, I must confess this will be my first defender. 
As for my friend, they have have decided on an illusion/rad controller.
With your friend playing Illusion/Rad I'd still lean towards a Storm/Psy since you'll be able to supplement the somewhat weak control in Illusion with your own slows, debuffs and KB.
If it's just the two of you then any convenient geometry will do to keep enemies chain-KB'd and thus effectively held, but in larger teams with you'll want to learn the approximate distance your KB does and get good at positioning yourself so that you use KB to consolidate rather than scatter groups (you might also consider flight as a travel power, since it's much easier to work out the angles when all the angles are "down"). Of course, if the proverbial excrement hits the fan you can always cut loose and just create chaos to give the team a chance to rally, but that shouldn't always be your go-to move.
Probably the most commonly abused power is Gale. Great if used well, will make teammates want to murder you if used poorly. Hurricane's also one that takes a little getting used to until you learn where to stand to get the most out of it without causing issues for the melee folks.
Kin/Psy also has a lot to offer this pairing, so there's not really a wrong choice - Kin doesn't supplement the control as well, but it will get Phantom Army up a lot more often with Speed Boost and will greatly boost your damage. So in general terms a Kin wouldn't be as safe as Storm but ultimately might pack more punch. At the end of the day choose the one that sounds more fun to you.
Iscariot's Guide to the Tri-Form Warshade, version 2.1
I'm sorry that math > your paranoid delusions, but them's the breaks -- Nethergoat
P.E.R.C. Rep for Liberty server
I have a kin/psy and I hate how single target it is. It really doesn't get to utilize the buffs as well as an aoe set would. I'd say the same will occur with storm.
If you don't care about aoeing then you can easily ignore this post, but the aoe potential Defenders bring is so wasted on single target sets.
I regreted going psychic even if it is fun to look at. On the other hand my Storm/Elec is one of my favorite combos.
I have both, I prefer storm.
Fr + psy torn + psy scream = knocked down, -rech and slowed groups of opponents!
Tornado for av's, lightning storm for all sorts of situations... good combo.
Title says it all...On one hand, psychic is a powerful damage set, while storm would compliment it by disrupting and controlling mobs while I pound them.
On the other hand, kin would of course, beef up psychics already good damage numbers a well as team members. the recharge is nice too.
So, thoughts? Comparisons?