If you're talking about AMXModX Pawn and SourcePawn standard (not new-style), the only difference I know of is SourcePawn has a dedicated String type.
Code:
//AMXModX string
new aString[256];
//SourceMod string
new String:aString[256];
However, the API (commands that the language supports) are completely different.
As to which one is harder to learn, I couldn't say.
There's also the decl operator, but don't use that.
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