Formally Accepted a position, then got a call about a final interview for a "better" position.

First time poster here: I have about three years experience in financial services and had really terrible situations in my only two jobs where i got fired in both. I have gotten back on my feet quickly and found some new positions. Its been tough with two black marks on my resume. I accepted an offer in writing with a tech company that makes software for FAs, which is a slight change of pace compared to what i was doing and seems very interesting, also it comes with close to a 10k pay increase from what i was making before, around 50k. I then got a call last night from Fisher Investments for an investment counselor role that i had been interviewing for but had not heard back about for a while, hence why i accepted the other job and did not want to wait around. They would like me to fly out and interview and i accepted because i figured that doesn't hurt. The position pays significantly more (85k plus bonus) than i accepted at the other place. I am at a cross roads now. I am schedule to start the position i accepted very soon but am not sure what to do if i get an offer from Fisher. Do i even start the position i accepted? I don't want to leave myself on thin ice with my past.
The job i accepted is great and the company is absolutely wonderful.
Fisher on the other hand, i have heard mixed reviews, but it is over a 35k pay increase from what i was making. I would have to relocate for the position with fisher whereas the position i already accepted, i would not have to relocate.
Any insight into what to do: if i should start the job i accepted, and if i do, what do i do if i get an offer from Fisher. If an one has experience with Fisher that would be great as well, since another 35k might not even be worth my sanity if they are as bad as some people say.

Thanks so much!!!