Having read this thread a few times it's interesting to note that nobody has mentioned the standard size Fifty Fathoms with a blue dial in Steel . This version has a brushed steel case with a guilloche style dial and looks completely different to the dial versions discussed here.
From an individual point of view , the mil spec will keep its value better than any 45mm version purely due to its limited number run. But that for me is the only reason I'd go that route . If you have a particularly small wrist then this could also be a reason to consider the mil spec . Other than that I'd buy (and I own two ) the steel version . Buying a used watch can be a nightmare but in the case of a fifty fathoms with the 1315 movement , your pretty safe, as they are really tough movements that were especially designed for sports watch durability , and time has shown that Frederique Piquet did a fabulous job with this movement . Servicing by a watchmaker that knows his stuff isn't too expensive either so you can factor that into the purchase price of a secondhand watch, Brendan quoted me around 350 for a service on a standard FF last year.
The case in steel makes sense for a sports watch, the polished case with its rounded angles and soft curves fits very comfortably on the wrist and it's easy to sort out dents and dings if you get them, more easily than a brushed case, and certainly more easily than titanium, which in the FF new version is grade 5 titanuim, which is harder refinish than steel, but tends to pick up small marks easier than steel . The brushed effect is more subtle, and some will like that, but again personally I like to know I've got a watch on, I like the weight of the steel over the titanium and I prefer the size of the 45mm version over the mil spec .
When I tried the mil spec I thought it wore very small, I was chopping at the bit to buy a used version off yonsonn of this parish, but I went to the boutique to try it one first, and compared to my standard version it just felt too small on my 7.25 inch wrist . Plus it's a dive watch, it should be big and bold with a big easy to rotate bezel, and easy to read, the bigger size of the standard watch fits the bill on that account .
The lack of hex screws on the mil spec and the lack of hacking are two other issues I think should have been included , and personally I'm not a fan of the moisture disc, but understand the heritage of that inclusion . It's should be remembered though that hex screws do present another issue of strap changing, which will take longer on the standard version, but in all honesty, have owned a few of these FF models, much like Tony ( LTF ) , I rarely stray from the sailcloth strap which is both comfortable and very hard wearing . I find the bracelet uncomfortable and it also lacks micro adjustment which is an annoyance on such a premium product . It does also make the watch slightly wider as the end piece spread the watch across the wrist further than the sailcloth does. Also fitting any other strap rather than the sailcloth can be hard, even a NATO will prove harder than you'd imagine , but it can be done .
You may also want to consider the antimagnetic property of the standard version over the others, for some this will not be an issue and the new versions do have a A magnetic hairspring, so you do get some protection and a display case back which will be a consideration for some, but again the standard version just does what it says on the tin, a no nonesense watch that's hard to beat.
If you want exclusivity or have small wrists but still want a FF's then the mil spec is the way to go, otherwise I'd buy a secondhand standard FF's . The titanium version will be harder to get on the used market so I guess that's just a waiting game . The FF is an iconic watch in all versions , so whatever route you take you'll still have a fabulous timepiece .