Ditto bhbarto's comments on Japanese fines being quite fine. I've pasted some photos below for reference. I've been through the exact dilemma as you, and hopefully I can provide some insight. My first question would be how you like your Pilot Metropolitan fine?
I found that the Pilot VP in Fine was a bit too thin of a writing experience for me. It's an incredibly smooth experience in both fine and medium (and broad for that matter), but I was not a fan of the way the fine wrote. I'll try to explain why: The nib is incredibly responsive and smooth, but as noted, it's incredibly thin. Thus, any variation or shake is immediately evident. It felt good but didn't write so well. I liked the fine nibs on the TWSBIs as they are stiffer steel nibs, so the feedback still allows for a cleaner line. For Japanese gold nibs, I seem to only like when they have a bit of feedback, such as on a fine Platinum 3776.
Ultimately, after trying the VP in fine for about a year, I swapped out for a medium at the DC Pen Show. To be honest, if swapping wasn't an option, I would have sold the pen. I really tried to love it in fine.
Thank you for this information. Do you find that your VP Medium is smooth?
The VP Medium is incredibly smooth. Mediums will always feel a bit "smoother" in my opinion given the increased surface, and the pen runs on the wetter side regardless of nib. It seems that your experience is with fine nibs, and TWSBIs are quite similar in feel to a Japanese fine steel nib.
One last piece of advice: it's pretty easy to swap out the nib and feed, and a lot of vendors will sell the nibs for each one separately. You could probably find a vendor willing to swap (either at a pen show) or you could actually buy both sizes depending on your mood. If I had liked the VP fine more, I would have done that. I actually have two TWSBI 580ALs but with a broad range of nibs that I swap out depending on my mood (or whatever else I'm carrying).