I play both Modded and Vanilla, however I prefer Vanilla. I usually play a modpack when it first comes out, and I will play it for about a month or so. But I get bored of them so quick, they are either so grindy that all you're doing is mining, or so easy that you are done with everything in the pack in a week. I often get bored of them too, because I'm a builder and I like it if other people can see what I build. With modpacks there are almost always no public servers available. And the ones that are public have banned certain mods and are very crowded and laggy. Out of all the modpacks I've tried I liked the Mindcrack FTB pack for 1.5 the most. It was grindy, but it had a tipping point where your grinding turned into machines doing it for you. I loved that. I just wish it would have had more building mods.
I like vanilla because I can focus on what I love about Minecraft, building and I can do it on friendly public server with no lag. People can see my builds and I can see other people's. They can help me with redstone, I can help them with building.