Upvote:2
To give a precise answer you will have to compare the fine print of both insurance policies and those will likely be different from car rental company to company and website to website.
However indicatively I found the following to be the case:
Of course when picking up the car the agent will point out these differences and more or less aggressively try to sell you their insurance. If you can live with the above risks/disadvantages of your pre-purchased insurance (you might need a print-out as proof) then you are fine. If on the other hand you could never afford e.g. not having $1,200 for a year because secondary insurance is slow in paying you out, then the more expensive option might be in your favor to avoid risks.
But again, it all depends on the fine print, there is no general answer IMHO.