TWSBI Diamond 580 - Medium or Fine nib?

[LandyVlad]

BEFORE you start throwing things at me for asking a question which has likely been asked many times, I'm new to TWSBI (and fountain pens in general).

 

I'm looking to buy a TWSBI Diamond 580 in "Iris' because it looks great.  But it's not cheap so I kinda want this one to be the one I'd use for my day to day writing.

 

Now, I have an older but basic Parker fountain pen, uses a converter, steel nib noted as 'medium'. That's all I know about it. (Aside from it gives a little too much feedback for my liking) 

Writing on normal 7mm ruled notepaper (nothing fancy) using Parker Quink ink (all I currently have, that'll change).

The line width, for what I'd call 'normal' handwriting size sometimes gives me closed loops on the e's.  For that reason I thought PERHAPS a fine nib might be a better option? 

On the other hand, it wouldn't show off a nicer ink as well as a medium nib would.

 

For what it's worth I intend to use Diamine Inks, primarily.

And while I'll practice on cheaper notebooks and copy paper I do have a couple of pads of Rhodia and Clarefontaine paper 'for best'.  

 

Maybe those with experience of 580s and both M and F nibs using Diamine ink could comment as to whether the TWSBI M writes toward the Fine or Broad side of things (if that makes sense).

 

If anyone is able to give me any guidance as to what else I might do / check / think about to help me decide between a medium or fine nib, I would very much appreciate it.