We typically travel to Orlando in mid-October, and have found that the crowds have been increasing on every successive visit. Many attractions used to be walk-on during weekdays in October, but now you can expect 30+ minute waits for the popular attractions (Harry Potter, Spiderman, Hulk, Transformers, Mummy, Rip/Ride/Rocket, Men In Black, and Despicable Me) during the same time of year.
It might be worthwhile to gauge the crowds and purchase UE when you get there if you feel that the lines are too long. If you don't want to purchase UE, you can always take advantage of the single rider lines, which are particularly helpful for Harry Potter and the Forbdden Journey, Spiderman (when the SR line is open), Men in Black (you can walk through the baby swap to re-ride if you don't want to walk all the way around), Rip Ride Rocket, Mummy, and Transformers. This assumes you don't mind splitting your family up for the vehicle-based attractions (you typically end up going through a similar times, and sometimes even in the same vehicle). It really depends on how you feel about your kids riding alone, but if you are, it can save a lot of time (like being able to ride HPFJ 3 to 4 times in the time it take to ride once through the regular line).
If you decide to stay on-site, it's a completely different story because the perk is unlimited.
Based on your length of stay, the cost of purchasing a 2 park Express (for a family of five) and staying off site would end up being close to the same amount if you stayed at the least Expensive hotel, which is the Royal Pacific.
I can tell you from experience. On-site = a lot more time do other things other than waiting in lines. Worth every penny.
Walt Disney World
Tokyo Disney Resort
Theme Park Insider Books