I would suggest the same, medium will be a smoother experience. Some people new to fountain pens find the finer nibs "scratchy" until their technique settles.I have the Metro in fine. It writes quite a bit narrower than a “normal” ballpoint. I mostly use it for writing notes in book margins.
For your first pen, I would recommend the medium, unless you have really small handwriting, plan to write mostly on non-fountain pen friendly paper (Moleskine, copy paper, etc), or need a really fine line for drawing/drafting/etc.
John's giving, as usual, some great advice here. For me, I've given away my Metropolitan as it was just a little on the heavy side and the grip felt a bit awkward. Glad I didn't buy a couple of them before I used the one for a good period of time. BTW, the person I gave it to just adores the pen, she's very happy with it.The Pilot Metro is an excellent pen, but its not the screaming deal it was a couple years ago when you could get it for $10. I would recommend sticking with the first one for a while, at least a couple weeks, then deciding what you do and don't like about it. If its perfect, than congratulations. But you might find the grip is too narrow, or body to short to use unposted, or you would rather have a screwdown instead of click-on cap, or on and on. Then, you might want to look for a second pen with the features and nib size you think you'll like. I would also point out that other pens, such as the afore-mentioned Safari, are much easier to swap nibs. Though Pilot does make a great steel nib. Anyway, just a few thoughts. But if you want to spend your money acquiring Metros with every conceivable nib and color, please knock yourself out. We all enjoy the hobby our own way.
BTW, the Metro's aerometric converter is probably my favorite feature of the pen. Holds a good amount of ink and is SUPER easy to clean. It is perfect for projects where you want to change inks a lot and/or don't need to worry too much about monitoring the ink level, like calligraphy or writing greeting cards. I wish it wasn't proprietary and could fit on my other C/C pens. Guess I need another Pilot.