Build... On your knowledge of building before you decide.
I have been in the same boat as your are now. It's a tough choice because 1) time is valuable and building takes a lot of time, 2) there are flying RVs for sale for less than the cost to build, and 3) there is an unknown that goes with building, which is "will I complete the task and do it well?" Here's the way I addressed each of these:
1) I realized that I've spent a lot of time doing fruitless things in the past (i.e. watching TV, playing video games, seeing movies,etc.). While they were enjoyable, they were short term forms of entertainment and didn't really change me in a positive way. From what I've heard from others, and experienced so far myself, building an airplane can be a life-changing experience, bringing joy, challenge and fullfillment all in the same package. Sure you could "buy and fly," but it won't have the same meaning.
2) Understand that if you buy someone else's airplane, it's never really yours. You can't make modifications yourself unless you are the builder (signed off as the A&P for that plane) or a certified A&P mechanic. Building is challenging and not for everyone, however, if you have more than just a passing ambition to build your own airplane, the build will be one of the most enjoyable aspects for you, in some cases (like my friend Jeff) people enjoy the build more than the flying! Also, maintaining your own airplane saves a ton of bucks.
3) This was the hardest one for me... You can never know what life will throw at you. Therefore, there's no way to know whether or not you'll complete the project. However, if you never try, you'll never know. Metal-working skills can be learned fairly easily. That's why we were able to spit out so many aircraft, ships, etc. during WWII. Attend a Sport Air workshop. It will help you learn, while dipping your toe in the pool as well. Also, go to Oshkosh! It will remind you that building and flying is a lot different than buying and flying..
Good Luck.