I have not compared the stereo vs mono on the same occasion, but I would say there probably is no difference in terms of sound characteristics like tonality, balance, texture, etc. I bought monos because I just love the mono mono look (yes I'm vain) but also from strong recommendations from Andy Payor as well. I have never heard the original Antileons so I can't comment on that versus the Signatures.
With the monos I'm sure there would be added control on the speakers/drivers, and of course the ability to power any speaker in the world. If Gryphon monoblocks can't power any speaker in the world, I really can't think of any amp that could.
Anyway, good luck!