Which one holds more value?
They hold equal value; Reddit makes no distinction between Post Karma and Comment Karma
What is easier to obtain?
Neither, let's try to explain the situation better:
Post Karma: Create a new account, try posting in top subreddits, and your post gets removed. Then, try posting the same content in smaller subreddits — how much karma can you get? 5? So, obtaining Post Karma is quite challenging
Comment Karma: Find the right subreddit, craft your response, post it, and there you go. Come back after a day, and your comment has 2 karma. Try again, and the next day you have -5 karma. It’s not easy!
What's harder to achieve?
Take a test, post 3 photos and 3 comments. Photos will likely garner a Karma of 0 or more, while comments may range from 0 to negative Karma. This means that posts cannot go into negative Karma, whereas unpopular comments can plummet to as low as -100 Karma.
Gaining Comment Karma for a Reddit account is much more challenging; it's unlikely that your comment will be noticed and upvoted, so you have to hope that the comment is positive.
Why do many websites offer 100 post karma for $10 while you only offer comment karma?
The question might be, why don’t other sites offer 100 comment karma? The natural progression for a Reddit account is to start by commenting, reach a good level of comment karma, and then begin posting. Don’t you think an account with 100 post karma and 0 comment karma is unnatural? …..and how were those 100 post karma obtained?