New job and Current Job need advice.

So I am currently in a bind between my current job and a new job I just got hired for. I was hired at my current full-time job as a temp or contractor about 2 months ago. I know I must be there for atleast 90 days to get hired on in the company with an actual position and better pay and benefits. The only problem is that it's back breaking work literally moving water heaters.. warehouse stuff. The other job which I have an A.A in Criminal Justice for and almost done my B.A this year is door to door serving court documents etc. I was hired as a Private Process server with a law firm. Of course, I would love to be hired full time but they said that you start off part time and there is room for a full-time opportunity based on the amount of workload in my district. I am stuck between taking this job opportunity in my field and the current job which isnt in my field but they have great pay and benefits. I'm not sure if I should quit my current job and take the job in my field or if I should test the waters and work on the weekends as a process server and stay at my current job during the week. I have never had a job in my field of study and I have never managed 2 jobs and being a full time student.

Thanks for your advice