Conditional as firm & Unconditional as insurance

Yes so if you dont get the grades that your conditional has specified, you will go to your insurance. That's why the insurance is there- if you don't meet your firm offer. (Similar to if someone had 2 conditional offers for firm and insurance. If they didn't meet their higher offer for their firm but did meet their offer for their insurance, then they would go to the insurance)
Regarding the economics question, a lot is in play here so there's no definitive answer. Looking at the university rankings, Bristol ranks higher than manchester for economics. But you also need to consider the courses both universities offer and see which one feeds more into investment banking. Also, do look at placement year opportunities at both unis and see what they offer as this can give you a headstart in the industry

Thanks a lot for your detailed explanation. It's difficult to choose from higher world ranking (Man) vs higher subject ranking (Bristol).

Then pick the one that has the most insteresting course and you can see yourself enjoying the city.

Just to add that if you don't meet your Firm choice, but they decide to accept you anyway, you'll be placed there and your Insurance offer will be removed. You won't automatically be placed at your insurance just for missing the offer, it's still up to the firm to decide.

Thx for your clarification. I wish i can get the firm even i miss the requirements.