I have a WDS Trek Mamba 29er and love it. However; it depends on what you plan to do with your bike.

29ers roll over anything but aren't exactly agile. They descend beautifully down hills and go up hills fairly easily. However; since I haven't ridden one on a true trail, I don't know how well they roll over things like rocks and logs. They sit higher giving you a better view and I can outrace DH on his bike. Landing is a bit harsh with a bigger bike.

26ers are easy to get and easy to find. You don't have to look for tires as they are standard sizes. They are more agile in turning corners and around tight spaces. DH's bike could easily overtake me on corners and in tight turns.

After being on a 29er riding a 26er makes me feel like I'm pedaling nowhere fast.

Because most of what I do is on gravel roads and not true trails, the 29er is perfect. I can hit 35-40 down hills depending on the grade and not have to pedal so much to get speed.

If your friend will let you try the bike out where you ride, do so and see if it works for you. If not, don't worry about it and get a 26er.