Hey! Blitz/Thresh main here.
Thresh and Blitz, while both being the hook kings of bot lane (Sorry Naut, you're a bit too gimmicky for my tastes) actually each have distinct playstyles that make them fun to play for different reasons. While for now I won't go into depth into their playstyles or even how to play them (It seems like you've already played them and that information may be a little unnecessary) I'll just rattle off a few reasons to buy each, and you can make a decision from there. While I could tell you who to buy, I am not you, and you may not enjoy the same playstyle I do.
Blitzcrank:
Pros:
Fast due to W movement speed, as such he stands a better chance of getting away with just running than Thresh
Actual mobility rather than disruptive CC to escape (Not counting Thresh's Q leap)
Stronger engage tool (Even the lowest of players know to focus a squishy target you've hooked, as it pulls the whole distance)
Once you have mastered the grab, the rest of the kit is very linear and easy to play
Melee for easy Relic Shield procs
Can hook people back to you over walls
His CC (except for silence) cannot be cleansed
Tanky from shield barrier, especially when buying tear, helps for surprising an opponent expecting an easy kill
Surprisingly strong early duelist if you are able to save the passive damage from the ultimate over time, combined with W and E steroids
Cons:
Not much disengaging options when it comes to being outnumbered in a gank
Chance of hooking an enemy tank and losing the teamfight for them
Thresh:
Pros:
Extremely potent at countering such as in a 3v2 ganking scenario, or even killing the gankers due to high base damage
Better at helping teammates escape through the lantern
"Ganking power," should you choose to roam you have many options to lock down a target
Can escape by hooking a jungle camp, which Blitz cannot
Two-pronged defensive and offensive utility
Can transport yourself and an ally to a hooked target (over a wall, etc)
"More CC" due to having more relatively long-lasting stuns and slows than Blitz, who has a long cooldown displacement and a somewhat inconstant knockup
Surprisingly strong duelist early due to high base damages, primarily empowered auto attack and Flay even when you continuously attack
Cons:
Cannot escape overly well unless there is a jungle camp to hook
Hook windup time making it easier to predict
Minor con, not being melee so harder to trigger the standard Relic Shield buy
Hope this helped at least a little! Give both these champions a critical eye and don't just get one out of peer pressure. :)