I had a 312 for a year or so and found it to be very top heavy and uncomfortable after several hours on the wrist. I would spend the first 10-15 minutes after I came home from work each day rubbing my wrist to get the blood circulating again, the relief from taking it off was palpable. It was enough to put me off the 44mm contemporary 1950 steel cases utterly. I replaced it later with its titanium twin the 351 which I adore, but as far as I'm aware there isn't a basic titanium 1950 GMT, a slimmer four digit steel GMT or Due GMT yet. There again you may not be a "weak girly-man" like me and simply not notice the weight as many of my peers seem to love theirs without reservation.
The steel 44mm Bettarini case I view as a masterpiece by comparison, heavy yes but never uncomfortable in my experience. For me it still embodies the original solid tool watch ethos that Panerai has arguably lost or certainly greatly diluted over the years of its climb to luxury brand and status symbol.
To be brutal I'd say that neither are particularly sought after, both are standard production models of many years and so will never be in short supply and the market is stagnant at the moment due to them flooding it with far too many watches and hiking up the prices since they began making their in house movements so unless you really love and need the GMT and can find an exceptionally good deal I would stick with the 24 you already have. It's a classic.