HI Andy, nice to see another fellow Alpher player on here
I am not a theory driven bloke so cannot comment too deeply I'm afraid, however in my opinion, providing your left hand technique can adapt to the different spacing and extra width, there is no negative in having a 5th string. All it does is give you an extra 4 notes and tons of positions. If I was starting to play bass all over again, I would have choose to start with a 5 string.