The current perks of staying onsite--and who knows what they'll be by next May?--are:
(1) Early Theme Park Entry. Entry to all 4 parks 1/2 hour before official opening time. Very helpful for rope-droppers.
(2) Ability to purchase ILL$ at 7 a.m. Off-site guests must wait until official park opening to purchase these. If you want to ride RotR and are willing to pay for it, there are often zero ILL$ available for this after 7:01 a.m., much less being available at park opening. It does happen, but it can't be counted on. And by the time you'd get to the park for RD from off-site, the standby line for RotR would already be pretty long since onsite guests get there 1/2 hour earlier.
(3) The usual perks of onsite, the main one being the WDW transportation system. At MK, the bus from your resort drops you close to the entrance. Off-site you'd be at the TTC and have to ferry or monorail to MK. It takes a while. Also nice to have bus transportation for the ride back to your resort in the evening. And if you like to take a midday break, being onsite is very helpful.
(4) This is just my personal perk: I love the theming of most of the WDW resorts. It's part of the fun of vacationing there.
AFAIK, there's no difference between onsite and off-site for making ADRs. It's the same 60 days in advance.