You might well find that you can do a bit of bargaining with whoever you choose, to get the same interest rate on your loan. But don't go Standard as they were recently rated, along with ABSA, as being the most expensive bank in SA.
I've been with FNB all my life, but they wouldn't give me a home loan - Nedbank would! So as much as I'd rather have all my accounts/loans with one bank, I'm sitting with two. Actually, FNB p*ssed me off royally about two years back - approached me, asked if I would be interested in switching my home loan across to them, I said of course, yes, filled out all the paperwork...and then they decided they didn't want me after all, lol...
But look into it - as I said, you might well be able to get the same interest - after all, at the end of the day all they want is to secure yet another person they can get money off.