Is it possible that SM might have "soft openings" a day or two early, or is 9/25 a hard date?
The HMH will be busy enough, but I'd bet money that SM will be insane. Regular standby wait times regularly surpass 60 minutes during the summer, and FPs can easily be all gone by 2 PM. This will be a 5-week (or so) promotion, so there won't be any saying "Hey, I can see it in a couple months, once the crowds die down". Locals in particular seem to love to visit an attraction as soon as it opens, and having one that is in a limited timeframe will just exacerbate that. I'm not trying to exaggerate or stir up concern, and hopefully FP will keep things from getting as crazy (though you'd definitely want to snag those early in the morning). If not for FP then I'd think there would almost certainly be lines going down Main Street...even with it, I wouldn't be surprised if that happened.
Isn't Friday the scheduled opening? If so, I would definitely arrive at the park early on Friday, as a lot of locals will be working that day but showing up later in the afternoon or evening. If you are hoping to ride SM multiple times, then I'd personally even CONSIDER bucking traditional logic and using MM on Saturday or even Sunday, so you can have a better chance of doing SM more times without as crazy of waits, and a way to get into the park before the crowds set in. If you're by SM right before the park opens to the public, you can even get a FP before the crowds reach it! And even if you arrive at MM 10 minutes before the park opens (though I do NOT recommend that as a strategy!!!!), you'll get in without a line, while there could be some pretty darned long lines to get into the park through the regular entrances.
At any rate, yes, I would definitely arrive early every day if there is any way in the world to manage it, especially if SM is a high priority. The HMH might be a bit problematic but it offers FP and is high-capacity, and will be running through Christmas, so locals/regular visitors shouldn't be quite as crazy about it, IMO.