I recently tried Deluxe Materials Perfect Plastic Putty as an alternative to Squadron Green and found that although it was very good for certain tasks, it wasnt as versatile as Squadron Green.
The two main problems I've found with it are that it's almost runny - I'd guess its consistency is mid-way between Copydex and PVA - which makes it difficult if you need to build up any kind of depth. I assume that to get that consistency must require a fairly high liquid content, which cannot help shrinkage rates as it dries (shrinkage wasnt relevant with the uses I've put it to, so wasnt noted)
The other problem is that I found it preferred sticking to whatever I was using to spread it with, rather than whatever I was trying to spread it on! That is a problem it shares with Woodland Scenics Foam Putty, which again I found excellent for certain purposes but even less versatile than the Deluxe Putty.
I'm still searching for a replacement for Squadron Green that I'm entirely happy with. Everyone who's used it speaks very highly of Tamiya filler but I cannot find anyone who stocks it locally and being a chemical means P&P charges are steep if ordered on line.