Both are nice boats and you would be happy with either one.
The oil injected 50s will be a lot lighter on fuel than the big foot 60s and are probably a bit more "green", the big foot gear box has proven its self.
In the end it comes down to personal choice and which layout you want, look at the condition of the boats and the motors along with hours and so on.
Take R10000 and crumple up each note, set them on fire and watch them burn, if it doesn't affect you and you didn't cry then you ready to own a boat