Admiral is a old established carrier. I have brokered some loads through them. They are a solid company. I can't tell you about leasing to them. It is difficult to find an owner operator who is leased to Mercer who isn't happy with them. I have met a number of them over the years. Most have high regard for Mercer. Their brokered loads are not all that great, for the most part. I don't recall how well they pay their owner operators. Since so many of their owner operator are happy with them, they must pay fairly well. Are you wanting to pull a van or flat?
I've heard good things about Admiral Merchants. One big plus is that they don't advertise for o-os in "the books." I have always found that the companies with the happiest/most well-paid contractors don't have to spend a bunch of money on advertising. They get good contractors by word of mouth instead.