It really depends on what's "best" for you. For us there is no way off site can compare to staying with Disney. Last trip we spent one night with friends at their off site timeshare. Nope....Not for us. It might be for you though. DVC makes the most sense if you like to stay in deluxe or better accommodations right in the heart of the action with the benefit of WDW transportation and perks.
onsite timeshare is for us......the major factor for us is the no worries, no hassle transporatation as we tend to spend the entire vacation on Dis property.
Although, we do plan to do Seaworld this time around we will use Tiffany Towncars.
If we want to visit attractions other than Disney, we do it on the weekend before or the weekend after our WDW stay. We pay cash for a hotel in a convenient location and avoid the higher DVC points for Friday and Saturday nights (If we spend the whole time at WDW, we pay points for the Fridays and Saturdays - I'm into the part of my life where convenience is pretty important, LOL).
That said, where is it written that DVC members cannot stay at a DVC resort if they visit something besides Disney?