It depends on what you want really. We took the view that we wanted to maximise our time in the park so did say on site. We also paid for reserved fireworks viewing (which I'd recommend to anyone!), character dining on one ocassion, and for magic express (paid luggage transfer)
We took the view that as the park tickets were expensive we wanted to make the most of them and that we were "saving" money over buying an additional park day. For example people often lose several hours waiting in position for fireworks, we would have lost several hours taking our luggage to the hotel and walking back to the park on check in, and check out, we gained an extra magic hour each day, and never spent any additional time queuing for characters.
For us that probably gained us a whole day of park time, without the extra park tickets, overnight stay etc
The bonus (other than luggage transfer, and magic hour) of the hotel is being able to split your day. Especially in the summer, the park opens at 8.30, the fireworks aren't till close to 11 pm and it wss roasting hot so our younger kids couldnt stay out all day, and benefited from quiet time in the hotel or a swim. We saw lots of people struggling with over hot, over tired kids because they weren't able to take a break. Equally when we went in the winter it was easier as the days are shorter but we wanted to go warm up/ escape the elements
If we had been just adults I'd be more likely to stay off site