Would help to know a bit more about your build (ideally, to see a Mids build if you have one made). That said, here's a very simplistic primer for you (keep in mind this is a generalization and as such there can and will be exceptions).
Ageless - gives you additional recharge, always nice for a Brute, and would be my automatic first choice if you're building Defense from IO Sets in any significant quantity. If you're not using Sets or if you're going for more of a recharge-intensive build then Rebirth might be a better option. The Ageless Radial path offers Defense Debuff Resistance, which I take on all my melee chaacters who have any sginificant Defense from IO Sets. It's invaluable. If you've got a 32.5% or a softcap Defense IO build, I recommend this.
Barrier - good if you think you need the boost to survivability or if you want to rez nearby allies. If you're using a higher-end IO build then you probably don't need Barrier, but if you're on SOs it might be a huge help to you. How often do you die? If you're frequently having close calls and rely on killing things before they kill you, this might give you some additional margin for error.
Rebirth - if you choose this, take the Radial path. The additional regeneration can be very handy especially if you already have large amounts of Resistance and Defense to stack it on top of. ELA is a resist-based set and offers strong resists when fully slotted. If you're surviving just fine and you don't have a lot of Defense from IOs, I would go for a Rebirth Radial.
Again that's general thoughts, but I hope it helps.
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On ELA I like Barrier above all. Stacking resists on top of already high resists is very efficient (90% res takes half as much damage as 80% res, for example). Defense is what ELA needs, and the enormous burst values for ten seconds after cast serve as an efficient defense debuff counter as well.
Why not Ageless? ELA already gets powerful end management tools and recharge.
Why not Rebirth? ELA already gets a decent heal and regen.
Ok well I just unlocked my destiny slot and looking at choices. I run a DM/ELE/Soul Brute. I am desinding between ageless, barrier and rebirth. From people who have these how do you feel about these 3 in play. Thank you for your thoughts.