The end of the night is what makes having your own car, whether it's onsite or off, worth it. I have waited in line an hour just to get on a bus at park closing, and then have to ride squished and/or standing up. Going right to my car and driving right to my door is amazing after a long day.
We have done both offsite (Windsor Hills) and onsite, multiples times both ways. Here's who "wins" in our experience:
-access to Disney perks (EMH, 60 day FP) Onsite, but we've never used EMH and probably never will. The 60 day FP is a real thing now, though.
-room cost plus extra charges (parking, resort fees, etc) Offsite, even with paying for parking
-amount of space available (sq ft, number of bed/bathrooms, kitchen, etc) Offsite. We've stayed in 2-3 bedroom condos for less than a Value resort
-pools (slide, lazy river, zero depth entry,splash pad, heated, etc.) Can be equal, depending on where you stay. The pool at Windsor Hills rivals any Disney pool. And since we can only afford Values or Moderates at Disney, I'd say for our money it's better.
-dining (any meals included? Quality/price of restaurants on site? Access to grills, off site food, etc.) Onsite. This is the only thing I really miss about onsite is having a food court.
-travel time to parks (shuttles/buses, driving--how variable is Drive depending on traffic?) It depends on how far away you stay offsite, but I'm going to assume you're looking at one of the closer offsite properties. MK is probably a draw because you still have to do monorail or ferry when you drive. The other 3 parks, driving is faster than waiting for and riding buses no question. And at park closing it's a slam dunk.
-any resort activities Onsite, but I will say that this isn't a factor for me because we honestly never used any of these even when we stay onsite.
-standard hotel ratings/reviews--cleanliness, helpfulness of staff, in room amenities, etc. Varies greatly depending on where you stay. But don't believe the hype the Disney is the only place that knows how to cater to guests. They used to be hands down the best, these days their service isn't what it used to be.
-any extras offered at specific hotels Varies
-laundry (in room? Free?) Offsite if your room as a washing machine. We LOVE this!