At this point I would just be looking at everything as a learning experience. The sysadmin job is probably a good learning experience, but probably not for long. The other job offer didn't come back to you for three months, so maybe they aren't in much of a hurry. Perhaps you could stick around the sys admin job for a few months to learn as much as you can from that job and then make the jump. In any case, I probably wouldn't be interested in sticking with the ISP for long.