Tempted to just write you off as inexperienced but I'll try to explain a bit. First off, just because they both have a hook doesn't mean they are the same. Let's break down each - I'll be speaking in general concepts, if you want to exacts on their abilities you will need to look them up on the site or refer to the Wiki. But I'll describe the basic idea.
Blitzcrank
Passive: Gains a temporary shield based on Mana when at low HP. Goes on CD after.
Q: Skillshot hook, pulls enemy to him.
W: Speed steroid. Zoom Zoom!
E: Melee range knock-up
R: Passively deals periodic damage. Active silences in an area and deals decent damage.
Thresh
Passive: Pick up Souls for AP/Armor. Thresh does not gain Armor Per Level.
Q: Skillshot hook. Stuns the enemy and pulls them a short distance, twice, and then Thresh can choose to dash to them.
W: Throw lantern, shield an ally, they can dash to you by clicking it.
E: Melee-ish range flay that can push or pull an enemy a short distance. Damage scales with souls and AD.
R: The Box. Creates a pentagon of walls that deals damage and slows. Slows for 99% on first hit, half damage and slow for the rest.
There are some differences between them. Not just in how they build (Blitzcrank builds Mana, because his passive makes him incredibly durable for a short period of time) but how they play. They both initiate but in different ways.
With Blitz, your goal is to pick out their most vulnerable target (generally the carry) and pull them into your team. Then knock them up. Then silence them. And by then, hopefully they're dead. However this can backfire, too. If you grab the enemy Amumu and pull him into your team, he's just going to ult you, and you just handed the enemy team a couple of kills. If Thresh grabs the wrong target, he can just choose not to pull himself to them and leave them alone. But if he does grab a good target, he pulls himself to him, Flays them towards his team, and then Boxes them in with his ultimate.
That's part of his combo and that's why his Ultimate is an AoE box around himself, instead of being cast at a location like Veigar's stun. Thresh wants to pull himself onto the enemy team and then trap them inside. His ult doesn't stun, but a 99% slow means you aren't going anywhere. So while the enemies are frantically trying to get out of the box, your team can just drop their damage into the box and rack up kills. However since it is on himself, he has to put himself at risk. If he could just place it on the ground it'd be way more powerful because Thresh wouldn't have to put himself in danger. Blitz is usually not in danger because he pulls to his team.
Basically - Blitz pulls and shuts down one target. He's really good at that, but then he doesn't do a whole lot else other than running around and spamming /Laugh
Thresh, however, is about team-wide disruption. You pull onto one target but try to flay as many enemies as you can and pull them back so you can Box them in.
And all that isn't even including Thresh's lantern. It is probably the best Save-Your-Ally ability in the game. For Blitz to save an ally he can knock them up - which requires running up to them and thus putting himself in the line of fire instead - or he has to pull the enemy from afar - which again, puts himself in that danger. Thresh on the other hand can stand over a wall and toss out his lantern and pull his ally to safety. Or he can use it to bring an ally into the fight by placing it over a wall. There are all kinds of combos you can make with this. For example, you want to get into Dragon pit quickly? If you're on the back side, Thresh drops his lantern next to him. Then he hooks Dragon, and pulls himself in. Then his ally grabs the lantern and hitches a ride. Now you have 2 teammates over the wall into dragon pit. You can use this to set up a gank where the rest of your team comes through the river and you flank them from the pit.
So no, Thresh is not a Weak Blitz. In many ways he is actually far, far superior to Blitz. However, Thresh does require more coordination and communication with his team. His initiation typically endangers himself more if his team doesn't follow up, where its much easier to start a fight with Blitzcrank. Thresh, on the other hand, has way more options on how to engage or disengage from a fight, where Blitz can really only pull and punch.
is for when you graduate from 
.
Both are bullshit champions played only by assholes.