Based on my personal experience of working for 8+ years in product based companies and 7+ years in Service based companies in India, I think it makes sense to post this.

DISCLAIMER : All these facts are based on my personal observations. Whether you should join a product company or service company - which one is good or bad - entirely depends on your personal choice.

Here is a comparison of key aspects of working in these 2 categories of companies.
Please Upvote if it makes any sense.

The interview process - As this is a coding forum, let us start with this part.
Most of the service companies' coding rounds can be cracked if you are able to solve Easy level problems (as a college fresher) to atmost Medium problems (if you are experienced professional).
HR interview, resume drill-down, soft skills like Group Discussion, Debate etc are also part of interview process, and may even have more weightage than your coding skills !!

Product companies interviews are mostly standardized with more (or equal) weightage on technical aspects - hardcore coding, system design and atmost 1-2 behavioral rounds.

Salary : Product companies are generally hands-down winners here. If you manage to join a product company right at start of your career, you will generally command a higher salary than most of the service based companies.
For lateral hires (people with experience), it is a slightly different story. In service based companies, you may get a very good starting package say 30-40% more than your current salary, BUT be prepared for very poor hikes post that.

Appraisals/Annual reviews : Product companies are nowdays tending to move away from the strict 1-5 ratings (1 being highest and 5 being lowest rating, or may have some other fancy names for these ratings). But, most service based companies have strict 1-5 bucketing.
Service companies are notorious for rarely implementing the salary hikes etc on timely basis. Same may or may not happen in product based companies as well in times of recession etc, but is less common.
And, similar to salary part - product companies generally give better hikes/bonuses/stocks as compared to service based companies.

Politics/favoritism in annual reviews/promotions - it exists everywhere irrespective of product/service company. It all depends on what type of person your reporting manager is.

Work hours and timesheets : Product companies generally allow flexi-timings, and many service companies may also allow the same if you are not deployed onsite at client locations. But, there is a catch - service companies have strict 9 hours of work + 30 minute lunch break - that is minimum 9 hrs + 30 minutes in office. You need to complete 09:30 hours daily even if you have no work.
Even 1 minute of shortfall will trigger an automated alert, and may even translate into Loss of Pay or Forced Leave etc.
Your in-out times are captured by your card swipe at gates.
Not just that - you also need to fill your daily timesheet - meaning how many hours you spent today on a particular task - it could look like - 5 hours of coding, 2 hours of meetings, 2 hours of code review and design discussions etc.
Product companies usually do not have such policies.

Dress code - Both product/service companies usually have dress code for public events like client demo, product launches, office parties etc - but in general it is smart casuals.
However, service based companies requiring client facing roles tend to have very strict dress code - you may be marked Absent if you violate even a minor thing. Some service based companies may have such strict dress code irrespective of whether the employee is in a direct client facing role or not.

Job security - Generally service based companies are clear winners in this aspect. Product companies are usually quick to fire people with poor performance reviews, or even in case of any signs of recession. Service based companies are definitely a safe haven if you are looking for a long 10-20 year career there.
Even if performance rating is poor for an employee of service based company, that person can easily stay on bench, and keep getting salary. Getting laid off for performance reasons from a service based company is a bit uncommon.
Needless to say, company policy violations do have serious consequences in any company - irrespective of product/service.

Skill enhancement : Probably more scope of rapid learning in product companies. In service companies, there is a high chance of getting stuck in same project for years and hence getting rusted. However, even in product companies - such a scenario is possible for laid-back people who are not keen to learn.
In service based companies, it is not uncommon for people to spend decades simply doing maintenance work like restarting a stuck server etc. If anyone is stuck in such roles, then very possible that such people are getting rusted in terms of their skills.

Notice period - Strict non-negotiable 90 days for service-based companies. It is a different story altogether that the same companies prefer people with less than 30 days notice period when they themselves are hiring.
This 90 days may be a BIG problem if you are looking for a switch, as many companies will not even call you for interviews the moment you mention 90 days notice, or can even cancel their offer.
For product companies, it maybe anything from 20 days to 90 days, but is usually negotiable.

Work-life balance - It mostly depends on your personal definition of work-life balance. In service based companies, there may be a higher chance of working in client's time zones. If you are located at client's site, then not a big deal - but if you are working remotely, you may end up doing night shifts or some other abnormal hours, which may affect your health in long term.
For product based companies, usually it is possible to work out a meaningful solution for coordinating with cross-border teams - but again depends on your negotiation skills, and your manager.

Travel opportunities - Obviously more chance in service based companies if you are good. You might get frequent opportunities to travel around the world as part of onsite assignments - nobody can say anything about this aspect in post-Covid world.