I've found this spreadsheet of pen manufacturers and nib tipping sizes to be useful - especially if I have experiences writing with at least one of the two I'm comparing.
Guide To Nib Tipping Sizes [ nibs.com ]
A question: could you substitute Platinum with Sailor in your experience? I just got a soft fine Platinum 3776 Century Bourgogne and wanted to compare it to another of the ones listed.
Far as the fine tipping on Platinum Century 3776 (I had one with a soft fine before), it's comparable to the Fine nib on a Sailor (be it 14K or 21K), however Sailor does not currently make any 'soft' nibs (and not sure if they did since the 80s).
The Platinum Soft fine is a little bouncy but doesn't have as much give as say the Pilot Soft Fine, or the FA nib, and has a bit more feedback than either. But initial line with a light hand is about the same.
The biggest difference I would say between Platinum Soft Fine, and a Pilot (#5) SF, FA, and Sailor H-F, is the feedback (Sailor is closer to a hard pencil, Pilot leans a little softer). Also the wetness on the Pilot seems to be wetter (with Sailor especially on the 21K side feeling a bit wetter but not as wet as the Pilot).