I was in the exact same position last year. Both my PIs asked me to continue with a Ph.D at my current university in Colombia (after finishing my M.Sc). Luckily I have a very good relationship with both and I just told them that how I trully felt. I did both my undergrad majors (ChemEng and Microbiology) and my M.Sc at the same university and I really wanted to experience my PhD at a different university, if only to get to know different technics, skills and research approaches. They both respected and understood my arguments (they actually ended up agreeing with my choice to not continue with my PhD there and encouraged and advised me to apply to specific grad school) and we continue to have a very good rapport, we are currently working together and they gave me excellent LoRs (I ended up accepted in Harvard xD). My advice for you will be to be honest with your current PI and more importantly with yourself. Applying to multiple grad schools is very common, but I think that it may only serve as a temporal solution.
Best luck with your decision and with your applications.