If you have reservations about a job offer and you're happy at your current job, there's not really any reason to switch. More money is nice but working at a toxic company can really ruin your life.

I would keep the a proven good boss over one whose organization's bad reputation precedes them. Life is too short to out up with a shit boss, except when completely necessary.

On the learning front - an easy job can be a great time and place to do a bunch of self study, if you're able to make yourself do that. I.e. drop a couple hundred bucks on something like Cloud Academy, and use it in your spare work minutes.

1. 3 months+ is considered the probation period for you and your new employer - you can change your mind, too. Better more than in 3 more months.
2. You can just leave the short stint off your resume.
3. Check your local laws, but if you're headhunted you probably get additional protections (waive probation, extra severance, etc).

I can only speak from personal experience, but when I was just getting started I prioritised jobs that would a) teach me a lot, b) look good on a resume. Your first job exists to set you up for your second, better job. I feel like an ass suggesting this, but if you've got the basic skills, you really can get a good headstart by having harder, "grindier" jobs for your first few years.

With how competitive the current market is for job hunting, I would stick with the more stable company.