I have been vacillating between ISLisp and a certain style of CL for some time now. The crux of the matter is the effort involved in porting if it is decided to productise a prototype. I think these two Lisp dialects (and ELisp) are quite close, similar to dialects of 1980s BASIC. So porting should be manageable.
What is gained by not using CL from the start? There is some cognitive load from the more (non-essentially) complex language. And the temptation to go down the rabbit holes of learning this or that QuickLisp library, when for a prototype you should be strictly limiting functionality.
So for now I'm continuing with ISLisp and could port to CoreLisp running under Allegro/LispWorks, with full access to QuickLisp, in the future. Rejected options include: