i'd assume pc will have more options becaue it's 92% of the market.. maybe ootp sect of the market is a larger share for macs? if so, i'd still go pc unelss it's 50/50.
it's user submission, so play the odds on that stuff... think nearly all of it is interchangeable? so, read into mod forums about hte stuff you like and it may not be a concern... buy the one you prefer
(and, i despise macs, lol.. trying to be fair and balanced, but probably a little like fox new's version of fair and balanced
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since most of the custom mods are just picture files and such, i'd assume it makes no difference which you choose... as far as bugs, again, macs are a fringe share of the market... but, if they offer it to macs, they will most assuredly work out the bugs, even if it takes a bit longer. (read there's a current crashing problem, but if it's real and fixed in next pathc, that's the same process a pc would have to go through too.. .so is there a real difference? no idea)
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yes, you can easiyl expand, and hold an expansion draft for the new team(s).
you even have real amatuers for a ~few years. teh expansion draft may tap into them if it's like an 'inaugural' draft.
search ootp19 manual for "inaugural draft" that settings/manual page also has the expansion draft funciton and you can read more details from the horse's mouth. i've never expanded or used that option, but i've used a FA draft and an inaugural draft before.