SRXs are a completely different animal. 4 valve head v 2 valve, twin carbs/twin pipes v single, Balance shaft in the SRX which is evident. The only thing is that the SR500 has that "je ne sais quoi" that the SRX doesnt have. As the man says... you pay your money and take your choice.
My Opinion only, having restored 5 SRXs and 2 SRs (1 500 and 1 400)
HTH
Ian