Which one is better between ProductHunt and Reddit?
Well I voted for Product Hunt but both serve different purposes in my opinion. Product Hunt is best at preparing for your launch, finding like-minded people to connect with and actually make your launch visible. Reddit on the other hand is nice because some subreddits are used by a lot of people (r/entrepreneur or r/startups) so you can (smartly) advertise for your product of get feedback, etc
I went with PH because I feel like its functionality lets you reach much more people who can either give you feedback on your projects or even start using your products/services once you launch them. Also, from my experience people on PH are much more likely to stay on-topic than on Reddit. To be honest, though, I've tried Reddit for only a short period of time, but while there are many enthusiasts over there as well, I didn't find it that interesting.
Founder of Tetriz.io
It depends of what you are looking for. Reddit has a wider audience, and you can micro target the audience by focusing on specific subreddit. Product hunt offer an easy way to reach most its user: make a launch. That make Product hunt more time efficient, if you goal is to promote your product.