This is a very personal decision. I have done both, onsite and offsite. Onsite is great when one can afford a deluxe. I have had nice stays in moderates and don't ever see myself staying at a value as I would perfer to stay offsite rather than pay that price for a value. I have had great vacations offsite and great vacations onsite. I can take it or leave it. However, this is just my opinion, when I stay offsite I don't feel as much disney magic. If your looking to stay offsite and spend a fair amount at the hotel other than sleeping, then pay the extra money to stay at a nicer resort (Marriot Orlando, Swan, Dolpin, Hyatt Grand Cypress, Peabody - whatever). When I stay offsite, I am in the parks most of the day. We usually have one day or half a day where we may spend time at a the hotel or shopping in Orlando. During that day at the hotel yes we will swim, sit in hot tub, etc. but the pool and area aren't themed but are generally less used than the ones at disney resorts. There is give and take on both sides.
As for my next trip, I am on the FTP and at a moderate. I have decided to fly in the day before and enjoy DTD. As I have called Disney several times to inquire about checking in the day before but there is only the rack rate available and I don't think that the moderate is worth that. So I went on Priceline to bid for resort and ended up with the Wyndham in DTD. I am so happy and will be more than thrilled to spend my first night there. I would be thrilled even if I had a few more days to stay there.
Best of luck in whatever you choose.