I don't know so much about off site, but I have stayed on site without staying in a Disney hotel. Same reason price. We loved POR when we went there in 2008. Since we have been staying SoG. ....
Heck I can never figure out why military take the 40% off at poly, they still pay $350 or more a night, when I pay $105 or less, the room is the exact same in the two queen beds and sofa sleeper, size SOG is bigger, and every morning we walk the beautiful grounds of SoG and poly to get where we are going,to us we miss nothing but others feel it is not the same. But as I say our room is to sleep in, we don't hang out there, we don't ACTH tv, we hardly use the pools. So it is not worth the price of $250 to $550 more for what you get, but that is us. Most people can not understand how we can be in wdw and be in bed at 10 or 11!
Well, first, of course you know that there are relatively few people who CAN stay at Shades of Green. I will never be able to stay there. So bringing up SOG with such a question is tricky, because you might get someone to want to stay there when they can't. I try to avoid threads about SOG b/c it makes me envious.
And I imagine that military members might go for different resorts because their priorities are different and they want different things.
OP, my current version of offsite at WDW is staying at a Universal resort. So that's hardly a cost-saving measure.

But really, for me, being onsite at Disney isn't something I do for the perks, it's what I do for the actual hotel. We almost always rent a car during our stays, and have decided after our last trip that it will only be in the most drastic circumstances that we'll take disney transportation to and from parks. We'll drive or take a cab. We just don't much like the buses. We did DME last trip, and while it was OK, having a car would have been better. We don't have things sent back to our resort when we buy them, not after experiencing the way the Loews hotels up the road at Universal do it, where they actually deliver packages *to the room*, not just to the resort.
I stay at POR because it's beautiful. OKW because we love it. I have a new found adoration for VWL. etc. Our hotel stays are based mainly on the hotel, and also what we can afford, $ or Points, wise, at that time. Out at
Disneyland, we have bounced back and forth between off and on site, solely dependent on what we need on that trip.
In January I'm signed up for the half-marathon. That race is 2 weeks before a cruise we're already booked on. We had planned on a week in the parks before. So that leaves me in the area a week before DH can get there. Whoopsie.

We're still deciding what to do. Go to Daytona and stay with family for that week? Stay in a beachy hotel there and be near family? Stay in the area, perhaps at Windsor Hills for the first time? That would be really cool, actually. And probably the combo of cheapest AND easiest for all involved (with family would be cheapest but definitely not *easiest* for all involved).
I have no overriding philosophy or never-do-that-again sort of idea; I just go with what works for each particular trip.
Do what you need! Do what you want! I'm sure you know the perks (dubious though I personally feel them to be) of staying onsite, and if you would miss any of them if they weren't available to you. There are a gazillion options in the area, and many of them look INCREDIBLY cool. I mean, have you seen some of the slides while going along I4? They look amazing!