Only you can answer that question....and you can't answer it until you try it.
As others have said, split your stay. If you do not want to commit to an on site resort for your entire stay, then stay at your usual hotel for the majority of your trip, and then switch to a Disney Resort for your last few days, just as an experiment to see if you like it.
Then you will know if it is worth it to YOU for next time.
For my family, it is well worth it to stay on property.....we would never do it any other way. We love being right in the middle of the magic, and you just can't beat the themeing of the Disney resorts.
And it is so nice to just hop on the boat after a morning at the MK and be back at our resort, usually within 15 minutes, for our little ones naptime (we stay at the Wilderness Lodge). Then after some rest and maybe a swim, it is back to the boat for an evening at the MK.
That works great for us because we spend alot of time at MK. Sounds like you like to be at Epcot, so you could have the same experience if you stayed at an Epcot area resort (or even the Poly...you can actually walk to the TCC from there to get on the monorail to Epcot).
And we still rent a car, even though we stay on site. We drive to all of the theme parks except for the MK (since we have the boat transportation).
Whether the buses will bother you or not is also personal preference.
For my family, we have found that driving is much less of a hassle....but we have strollers, and we usually can't hang around the park late to let the crowds thin out because we have little ones who need to get to bed.
We have gone with a car and without, and we have found that we can get to and from the parks MUCH faster and comfortably if we drive ourselves (MK being the exception). Again, that is what works best for MY family. Some families prefer the buses over driving themselves....it really comes down to personal preference.
Also, there is budgeting to consider. Two weeks is a long time....and it sounds like you do this twice a year? We would not be able to afford to stay at the WL every night and rent a car if our vacations were that long. We usually go for only 6 or 7 nights. So again, it is hard for others to answer your question, because everyone has a different budget and priorities. If you can afford it, I really think you should try a Disney resort....but I must warn you....it might be REALLY hard to go back to staying off property after you do!