As a growth lead, I've extensively used Hotjar and Mixpanel for companies of all sizes, including those with millions of users and even small early-stage startups. Here's an in-depth but brain-friendly buying guide.
Quick Links
Getting a 30-day Free Hotjar Trial (*to business plan)
Hotjar vs Mixpanel for Dummies
Think of Mixpanel as a friend that can do math! While Hotjar is your more creative friend.

They aren't really that similar. Except that both help you better understand your users.
Hotjar even integrates with Mixpanel. You can send Mixpanel data to Hotjar.
This helps you layer on the numbers onto the visuals.

Hotjar vs Mixpanel: A Look Inside
Mixpanel overview
How does Mixpanel compare to Hotjar.
What do you see in Mixpanel that you won't see in Hotjar?
Different views in Mixpanel
Technical skills realistically required to use Mixpanel.
Hotjar overview (wrt to Mixpanel)
A look inside Hotjar
What is similar and what would be different than Mixpanel
Where Hotjar wins and where it loses (gaps in data)
A quick demo
Hotjar vs Mixpanel: Time to implement

Hotjar:
Hotjar is something your team can realistically implement in an hour. They make it incredibly frictionless that even a marketer can do it if they have access to Google Tag Manager. If you have a web app and a marketing site, it's far easier to track because you need to only paste a single piece of code.
Mixpanel:
Mixpanel also claims that implementing their tools takes as long as it takes to eat a burrito, which is not quite accurate. It takes weeks of active work and then a little work every month forever. The reason for this is initially, you have to pass every event individually with the help of a dev. After that, you're either working with them to add new events as you release new features. Or ask them to add/remove some events. There are products like Heap/Fullstory that offer auto-tracking every possible event but I have decided against doing it.
Hotjar vs Mixpanel: Pricing
Hotjar and Mixpanel both have a very generous free forever plan. I'd say Mixpanel is far more generous.

However, Mixpanel gets expensive much faster than Hotjar does, and it is harder to pause because you don't automatic session sampling like you do in Hotjar. Hotjar is easy to downgrade / upgrade without worrying too much.

Note: If you'd like to send integrate Hotjar with Mixpanel, you have to upgrade to a paid plan.
Hotjar vs Mixpanel: Usability?
Start with Hotjar if you are a smaller team and have little data.
If you are a UX designer or just a more creative person that prefers visuals over numbers, use Hotjar.
If you like math more and have high volume, use Mixpanel. But then send Mixpanel data to Hotjar so you have fewer debates and better alignment.
Implementing Mixpanel is a bit more intensive. You need someone on your team that can build an event tracking system.
Something of this sort:

It's not impossible to learn how to do this, but it's usually better to hire a consultant to make sure you're setup correctly. With Hotjar, there's almost no learning curve.
30-day free Hotjar trial
If you'd like to start with Hotjar, use this link to get a 30-day trial on their business plan which isn't available on their site and will be required if you have any custom integrations to do. Use this time to set it up properly. After that, you can either enter your card to continue on a business plan. Or, just downgrade to their free plan.
For Mixpanel, they have a startup program that gives you credits for a year. See if you qualify. If you know you don't qualify, there isn't a trial.
I hope this helps!
- Khushi Lunkad