Cindy, don't fret! Having a wheelchair will make it easier on you
and your family, and let you all have more fun. You won't be a burden on your family, they love you and want to see you having fun along with them. I've never done an ECV before, but instead have used a wheelchair the last... oh... about 6 trips I've been to Disney. I now have my own chair, and my mother had no problems helping me push around the parks when we went in September (and this was with a slightly funny knee, as well). Really, don't stress it, when you get there you'll realise how easy Disney makes it all.
OK, now for the practical stuff. I think most of the rentals tend to be folding wheelchairs. The advantage of these is that the can (rather obviously) be folded, so can be stored more easily. This is particularly useful for space-saving in the hotel room. Folding chairs tend to weigh a bit more than rigid framed, but as this is just for a holiday and nothing long term, I don't think you need to fret about that too much.
You can carry a wheelchair on if you want (some bus drivers will still want to strap them down, some won't), but be aware that they are quite heavy and the step onto the bus can be a bit high. I've tried both hobbling on and using the ramp / lift, and I have found that the latter is
so much easier. I personally would suggest using the help there is available to reduce pain, fatigue and hassle, but it may turn out to be more bother than it's worth for you guys. See what works best.
One last piece of advice (for this post, at least), if any of you haven't seen it already, check out this thread on
tips for wheeling Disney.
Good luck to all you first time wheelers out there, I hope you all have magical holidays

!