Bs/Inv or Db/Inv is a hard one to call. Broadsword, with its slower attack animations and activations would be gentler on your Endurance than Dual Blades which is extremely spammable. However Dual Blades probably better at quickly dealing with the large mobs that /Inv thrives on.
If you're expecting to solo more than team then Broadsword is probably the better choice. If you expect to spend most of your time in teams then I would favour Dual Blades.
I have a BS/Regen and a DB/WP scrapper (50/46 respectively).
I love them both.
Broadsword does great burst damage, it really makes the most of crits. For single target work its great. When fighting higher levels/other players getting a single crit can make a difference. It has a few PBAoE/Cone attacks, but generally its not worth herding around you, just concentrate on the 1 big bad with Broadsword. Downsides - its slow, but steady damage, in large mobs you will need a hand. Some of the attacks do knockup/knockdown.
Dual Blades is pretty much steady DPS, combos add some spice to the proceedings, great against mobs due to variety of small cones and PBAoE + your combos. Jump inside a mob, mash them down. Downsides - crits make only a wee bit of difference, any lack of accuracy will kill your combos so its not really suited for PvP.
Broadsword - Burst damage, slow and steady, crits = love, single target perfection.
Dual Blades - Constant stream of damage, good in mobs, crits aren't noticed, but combo bonuses can be great. Needs accuracy.
I hope that helps a little! 
I would say that BS has much more reliable big numbers burst damage due to having buildup and the fact that Disembowel has a 10% chance to crit anything but players/minons and Head Splitter has a 15% chance to crit anything in the cone range.
But you do get a little frustrated by the standing around inbetween animations though.
DB has lower cast times and animation times for most powers, but with no BU type power it relys on stacked combo bonuses which are less reliable. Also the DB power with the 15% chance to crit is a single target not a cone attack.
Want to smash hard single targets fast ? broadsword is your man.
Want to kill mobs? dual blades is your man.
i'd say with your taunt aura in invuln, DB has a couple of AoE's as part of Combo's so these would be very useful, especially with the sweep combo.
Defiant 50's
Many and varied!
@Miss Chief
I'm running low on slots, only got one left and I love all my toons so I can't delete anyone, and I find it tricky to find a suiting power for my newest creation Major Marvel, I was gonna make him into an ss/inv tanker, but I already have one and find it very boring to play under level 40.
And I think both Broadsword and Dual Blades looks very fun. [And powerfull.], so my questions would be: Whitch is the best one for dealing damage with the combination of invulnerability? Perhaps hearing about the pro's and con's of both could be good for making up my mind?