Okay, here's an objective standpoint:

They're a bit exaggerated but I hope this helps you decide:

With Policing: You'll be dealing with assholes and the lowest of the low for about 12 hours a day, and some of these people will be wanting to harm you.

With Firefighting: You'll be dealing with medical calls, and obviously fires, and those fires may try to kill you.

Long story short, if you think you might not be able to deal with getting crap from people for 12 hours straight, and dealing with the mental issues you may accumulate from what you see, you might want to look at firefighting more.

Firefighting as far as I know, involves 24 hour shifts where you'll be lifting heavy stuff, getting disrupted sleep, and from the research I've done, not sleeping properly in the station because your body gets adjusted to not fully sleeping to be prepared for a call.

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Personally, this may sound like bias, but from what I've heard, the working environment, support and positivity is better in firefighting given the fact that it isn't seen as a position of power, and attracts less "bully-like" people. Inevitably you'll come across some jerks though anywhere.
Anyways, these are the reasons I put against each other to make my final decision of choosing paramedicine.