Having started my company as a wedding/school dance DJ (in the same position as you) -- there's something to be said for the flexibility of the SRX-812P/SRX818P combination. However, if I was in your spot and looking to purchase a PA, I would actually get 4 SRX-812Ps and 2 SRX818Ps to start with. Since, as you know, at weddings the people who are looking to really enjoy the music (i.e. feel the bass) are going to be on the dance floor, that's the main spot where you need to have that low end coverage.
Put a white tablecloth over it, let her decide how to decorate it. Kids candies and snacks, photos, etc.
Not sure if they're going to want all of that stuff shaking off every few minutes.
But in all seriousness, brides can sometimes be as bad as corporate event planners in that they don't want to see "all of those boxes everywhere," and having a smaller sub that's easier to tuck under a table or in a corner is not a bad idea.
With two more SRX-812Ps, that makes a quick and easy ceremony or cocktail lounge system. Since the speakers have two inputs available, you can put the ceremony music feed into one input and the officiant's lavaliere microphone into the other, for example. Or, if you have a larger venue, you can use that second pair as delay speakers. (I started my business with a pair each of Mackie SRM450s, SRS1500 15" subs and SR1530 15" 3-way tops, and within three years I added 2 more pairs of 450s and a pair of SA1521s. And sold the 1530s after a year, when I realized that I just didn't need them enough to justify the expense. Part of these upgrades was to support having one or two other DJs freelancing for me, but mostly was to improve the flexibility and coverage of my setups. You can include the main pair of speakers in your basic package cost, and then have the second pair of SRX-812Ps be "Just $129.95 total more" or whatever you want.)
… and then buy one or two pairs of the SRX828Ps for the school dances, and you'll have a ridiculously good sounding system!