Welcome! We have stayed in 2BR/2BA villas both on-site and off-site.
As lacee says, the only way to make sure you can get into an on-site "home away from home" villa is to rent someone's DVC points. As you may have already seen, the going rate on points is roughly $10 per point. The number of points required for your stay vary greatly depending upon what time of year you go and what actual nights you stay (Fri & Sat always require more points). Although I don't remember the actual numbers, I think the last time we went it was going to require around 250 points to stay in a 2BR/2BA villa at BWV for a whole week. At $10 per point, this would work out to $2,500. However, BWV is very popular and we found it impossible to get this rate. We ended up staying Sunday through Thursday in order to economize a little and it still cost around $1,950.
We also have stayed several times (and will be staying again in March) at the Sheraton Vistana Resort. Each time we've stayed there, we've rented from someone who owns a timeshare week. On average, we have paid around $850 per week for a 2BR/2BA unit.
In terms of time, we have found that it takes roughly 15-20 minutes door-to-door to get from the Vistana into any of the parks. When we were at BWV, I know that it took us 10-15 minutes just to get to the parks that were within walking distance (Epcot & MGM). When we took the bus to/from MK, it took a lot longer. If you stayed at VWL you would definitely be able to get to MK faster, however, the other home away from home resorts will require you to take a bus to all of the parks. If you can't stand the thought of driving a car while on vacation, then there is a big advantage to staying on-site with respect to getting to/from parks, but I would argue you won't save a lot of time (and it will take longer in some cases).
The other clear advantages include access to EMH at the theme parks, access to UPH passes, and other little "perks" associated with being a WDW resort guest. If you think you might want the flexilibity of the UPH and you think you will take advantage of EMH, then you should be willing to pay a premium to stay on-site.
Now, you will get varying opinions on which one you should do. If cost is not an issue at all, then I would stay on-site every time. But obviously you could stay more than 2 weeks at the Vistana for the price of a week on WDW in an almost identical unit. It is very nice to be staying on-site, make no mistake about it. Some people wouldn't ever consider staying anywhere else, and that is great. To us, we found that the advantages of staying on-site weren't worth the extra cost and we will likely never stay at one of the home away from home resorts again.
