Another point about the two different a/c is you can't do the MEP rating in the diamond twin star if you ever want to fly anything else.
Codswallop! Differences training is required between any two MEP types, irrespective of what they are. If you do the MEP on a Seneca, you need differences training onto, for example, the Duchess the same as if you did the MEP on the Twinstar.