Old Pentax prime lens vs digital zoom lens

Hi. I am seriously thinking of getting an old Takumar 135mm f/2.5 lens but am not sure if this is a good move since I already own a modern Tamron 18-200mm lens. Can someone tell me if there's a noticeable difference in IQ between these two lenses, if the latter is set at 135mm and the available light is adequate? Will the former, being a prime lens, perform better (ie, sharper)? Comments and suggestions will be much appreciated. Thanks and regards.