GR can kill heroes from afar w/o triggering their self-defense skills or talents, but so can Hill Giant.
They are both good heroes. The fact is
Immortep: The more beginning to mid game hero. With help in braking walls for gems and stunning flocks of army camps while they are not upgraded.
Grizzly Reaper: The more a end game hero, with his skill 7/9 or higher to hit 5 targets is godly in the arena, raiding, Storm mesa and HBM.
So if your a new player with low skills focus on Immortep, if your end game with high level heroes (Or your P2P) I highly recommend Grizzly Reaper.
If*
You don't need a Sprint Talent for Reaper in order for him to be usable. You just need to know where to use him. He and Immo just are completely different heroes overall and the only reason those quite incomparable heroes ever were compared in the first place, is because they were both the most expensive shard heroes at the time.
Reaper is, like Charles Warmonk said, just the more end-game hero, since the whole breach-and-destroy - strategy won't work anymore, if you're getting up in the ranks. So he is very useful for destroying excess troops or high-level heroes without your team even having to be touched in the process.
And Immortep is more the kind of hero you use, when you go full-on dps. Logically you won't use any of them if you have any of the roll-only-heroes anymore, but Immo would be the first to give up. Reaper at least pairs well with Aries or TG. I always pair those three in raids, one in each corner. Then everything is dead until they get to the middle of the map.
Reaper and Immo are both not strong enough for directly being sent into high level bases with lots of roll-onlys. The direct approach will kill both of them. Just use Reaper for Raids, Dungeons etc. Therefore he'll always be usable, no matter what your other heroes are. The new features like Mesa or Lost Realm all play into his hand anyways. Ever seen someone with an Immo in Mesa? No? Well, guess what, there are reasons why Reaper is in his place here. And in higher level Arena Reaper is also more useful since he does have the higher damage proc. And 280 % of 4039 on max level are significantly more than 280% of 3232. The difference is even bigger considering that Reaper has the fastest atk spd and therefore procs more often.
Immo on the other hand is NOT the ultimate boss hero. Better than Reaper, but he'll be replaced first thing. In Raids he is good, but not the best, Reaper, Aries, TG and Vlad are better. And well, in Arena? As I've stated already, his damage count and attack speed are lower, meaning that in something like this, where damage is important, he won't be your end-game hero. Sure, people at the beginning of their game SHOULD farm for him first. But as you evolve in your game and pile up some shards, Reaper will be a choice and this time the one and only. He needs LOTS of shards put into him, 5/9 skill at least - not to be functional, but in order to be end-game, or at least take a step into that direction. Immortep doesn't get that far, no matter how many shards you put into him. With Reaper you can really see huge developement with every skill level.
I personally have to say, that since I have Aries, I don't ever use my Reaper anymore. And anyone who has Vlad probably kicked Immo off their squad anyways. But for those who don't have the luck to have either of them - why don't you use both? F2P should consider that anyways and from experience on smurfs they work together quite well. Even though Immo has a bigger tendency to die, if they go head-to-head. If you only got one hero base free, then use Immo if you're a beginner, and use Reaper if you're willing to put all your shards and books into him for a while, in order to at least make a huge progress in the same line.
But can we please all stop argueing about who is better? They're simply two different heroes for two completely different purposes.
Dawn-elena, just pointing out something you said,
Atlanticore is awesome, he was my very first legendary hero, and I didn't like it at first, but his cooldown is not something to be worried about. He is a two-times proc, (with revive talent, obviously) and thats enough.
I mean, when he is first line and procs the first time, the battle is done. Enemy procs will bounce back to them, while they get all the dmg from my heroes. Wait, you killed my atlanticore? Surprise, he revives with no cooldown. Very useful in dungeons, lost realm & boss fight.
The only places where I don't like him is HBM and SM.
Atlanticore is awesome, he was my very first legendary hero, and I didn't like it at first, but his cooldown is not something to be worried about. He is a two-times proc, (with revive talent, obviously) and thats enough.
Cool down is always something to be considered.
Team fights are decided in the first 20-30 seconds, which is 2 atlanticore procs with revive, and that's enough for him.
Cooldown is out of the question if his proc lasts about 5 seconds, dies and then come back to life with full proc bar and no cooldown. So yeah, half of the fight, if not all, is with his shield on.
The only place where his cooldown is something to be worried about is during boss fights & demon.