Which is the better stealth power?

Concealment or Cloaking Device?

If you mean Concealment pool Stealth or Cloaking Device, then Cloaking Device is better, though not by much.

It costs slightly less endurance and does not have the speed debuff.

Hm. Okay. What about detection by the enemy? Which is better suited?

I like to take Super Speed and slot in a Celerity: +Stealth invention origin enhancement. The concealment stacks (for 65-foot stealth) and you get the best two dimensional travel power. You can even run Super Speed while flying or teleporting (I don't know about jumping, but I assume it's possible.)

That much stealth will protect you from detection against everything except snipers, giant monsters, and some of the Rikti Drones.

All this for one power selection. Sounds like a sweet deal to me.

I've a DP/Devices Blaster. I like to sneak up and drop some trip mines in a mob of enemies. I also drop some timed mines, careful to make sure both go off at the same time. I use Stealth & Phase Shift. The PS is just in case they see me, they can't do a thing. At which point I unphase and let the trip mine blow up just as I unleash Hail of Bullets.

I don't think the Fitness movement powers take sets any more.

For me it's more that sometimes I want stealth but not super speed. Typically if I take SS it's one of my last throwaway picks at 47 or 49. A stealth IO here becomes useless when exemplared. A stealth IO in sprint can be used at any level.

Also, and this is a frivolous concern but one I suspect is not unique to me, sometimes you don't want to have the frankly ugly transparency effect that a stealth IO produces active on your character. Zero impact on gameplay, more of a niggling annoyance.

To add to what plainguy said, detector NPCs such as snipers and rikti drones will see you through any amount of stealth, whether it comes from stealth the power, invisibility, what have you. Fortunately most enemy groups either lack detectors or they don't typically spawn in most instances in most level ranges.

When you attack, are hit by an attack, or click a glowie your stealth suppresses completely for ten seconds. Once that ten seconds has elapsed your stealth turns back on, but if you haven't put sufficient distance between things that aggroed on you in the mean time they'll likely remain angry and continue to pursue you. Breaking line of sight is important if you get spotted and want to continue to be stealthy. It's also worth noting that some enemies, most notably old fashioned clockwork, never lose aggro and will chase you any distance once you have their attention.

One more point is that cloaking device stealth doesn't suppress completely. Even when you attack or click a glowie, you still have 15' of stealth. Hit the enemies with a smoke grenade and you can still sneak by or click a glowie without problem.

Just to clear up a common misconception: Snipers do not see thru Stealth like Rikt Drones do. Snipers just have a very high Perception.

Here's how Perception and Stealth work in game:

A mob's Perception is the distance in feet that they can detect you. Most minions have a 45 Perception which means they can only detect you if you are 45 feet or closer to them. Snipers have a 149 Perception which means they can only detect you if you are 149 feet or closer to them.

Your Stealth subtracts from a mob's Perception. If you have Superspeed going that grants you 35 Stealth. This means a minion can only detect you if you're (45 - 35 =) 10 feet or closer to them. But a sniper can detect you if you're (149 - 35 =) 114 feet or closer to them.

A Stealth IO provides 30 Stealth. So if you've got a Stealth IO in Superspeed, then you've got 65 Stealth. Since this is more than a minion's Perception, then the minion cannot detect you no matter how close you stand to him. However, the Sniper can still detect you as long as you're (149 - 65 =) 84 feet or closer to them.

I don't know what Rikti Drones' Perception is, but their Perception is not affected by Stealth. So your Stealth value does NOT lower their Perception. Knives of Artemis and some Rularuu are the same way.

Snipers, however, are affected by Stealth, they're just so Perceptive that it seems like they ignore it.