If your PIC have a USART and you need only 1 SerialInput pin, there's no advantage to use SERIN, SERIN2, DEBUGIN. Even if you need only 1 SerialInput pin, there's no real advantage to not choosing a PIC without USART.

With a built-in USART, the whole thing could be hapilly managed with interrupts, which it's not really reliable with SERIN... unless you send tons of 'preamble' NULL character or so.

Yeah, i'm from an old school