If you are an experienced engine builder, and can do quality work yourself, and can get some good deals on parts, go for it yourself. Otherwise, I believe the long block assemblies and complete engines being sold by the manufacturers today really can't be beat. It's not as much fun buying a long block, but at least the math has already been done for you, and your only question is what color spark plug wires you want to get.
I recently built a 308, that I know will dyno at 350hp, for about $1200. I could sell it for $3500 tomorrow, but it's not for sale. I had a good deal on the parts, and I've already got all the other parts I needed. I did all the labor myself, and feel confident this engine will run for 200k miles with nothing more than routine maintenance. So for me, this time, it was a better deal to build it myself, but that doesn't mean that's necessarily the answer for you, or even for me next time. It just depends.
Good luck with whatever you decide.
Take care,
~Chris